Hello folks here to share my experience with my first Subaru suspension install. First ive done many many installs on many cars ranging from fuel pumps gearboxes to heads all from Chevelles, to Ford Tractors to Grand Ams to my Legacy. I know there are excellent guides out there i.e. Fred Beans guide and a few on NAISOC but I wanted to make one that focuses on the layman, with tools that most folks have access to in a non-shop environment.
Before I started the first thing I did was READ, look, ask, sit in, read more, ask more, then I bought

Sounds wild I know but Im particular about my vehicle and too low or not low enough would irk me to no end.
I went with Tein H Tech springs + KYB replacement struts/shocks + 1/4 HDPE spacers. I arrived at this choice after seeing a few Legacys with this setup and talking to owners abou their impression. I like the drop and to me the ride isn't harsh but more planted.
Tools:
Spring compressors (you can rent these at most parts stores)
19mm socket and wrench (short and deep well or pass thru)
17mm socket and wrench (short and deep well or pass thru)
14mm socket and wrench (short and deep well or pass thru)
12mm socket/wrench
3/8 and 1/2 drive ratchets and assorted extensions (4",6",12")
6mm hex socket and key
5mm hex key (optional for front endlink removal)
Flathead screw driver
Pliers
Jack Stands
Jack
Table or bench (for assembly purposes or you can do it at the vehicle)
Gloves (optional but will save your hands trust me)
Cheat pipe (24-30 inch long 1.5 inch dia. optional)
Wood block
Scissor jack
Parts I used:
KYB specific:
Front Right Strut: 339096
Front Left Strut: 339097
Rear Shocks: 341353
Rear mounts: SM5214
OEM specific:
OEM Front Strut Mounts: 20320AG00A x2
Springs:
Tein H Tech SKS70-BUB00
Reused parts:
Conical washer x 2
Bump Stop x 2
Upper Spring Perches x 2
Dust boots x 2
Now the debate ive seen is that you dont need new mounts simply rinse and reuse, I agree for the
rears this is perfectly fine. For the fronts I disagree due the forces exerted over the life of that installed suspension.
If/when I do this again I will buy ALL components news, assemble on the bench and simply do a swap. This will save you a lot of time!
Lastly (and FIRST) SAFETY SAFETY SAFETY! If you dont feel comfortable or have room to safely work on your vehicle DO NOT! A scissor jack can do a lot for the occasional tinkerer...including severely hurt or kill you.
Start with the Fronts:
Removal and assembly:
1. Place the car on jack stands, open the hood and remove the wheels. Make sure to place a chock behind your rear wheels, never can be too safe. NEVER attempt this with your car solely on a jack.
2. First take a pic of your camber bolt orientation, this will help you to set the camber post install.

3. Remove the brake line and ABS cable brackets with s 12mm socket and extension

4. Place a jack under the knuckle and lift it slightly, you want to support the entire assembly not compress it.
5. Remove the three strut mount nuts. (I have a strut brace installed)

6. Remove the lower 19mm bolts (The top is the camber bolt). If you dont have an impact gun it's time to whip out the old cheat pipe. Use your 19mm wrench and socket with your 1/2" drive ratchet, slip the pipe over the handle and apply pressure until the bolt breaks loose. Then ratchet them both out!

Ahh always fun when the cheat pipe comes to play
Once both bolts are out, lower the jack and the entire assembly will drop out and you are done.

The knuckle will rotate slightly and hang in place. This is not damaging to your vehicle or affecting your caster.
Repeat these steps for the other side and remember to snap a reference picture of the top camber bolts for the reinstall in addition to removing the front sway bar endlink!!! If you do not remove the endlink the hub will not rotate and you cant remove the lower 19mm bolts. This is due to the opposite side exerting force (b/c it is still connected and unsupported) through the bar, to the endlink, into the suspension assembly.
Now we move indoors or where ever you plan to assembly the front struts.
1. Lay out your parts so you know what you have and can easily access it.

2. Get your spring compressors and place them on your stock springs. These springs are a hassle b/c you cant get three coils exposed enough to get the compressors on top of it or the recommended 180 deg. separation. Instead youll have to clamp two coils and crank them down to the ends of the threads of the compressors.
Use your 19mm socket and wrench to apply even compression on each side until fully compressed. Note that you dont have to compress them back into a tight coil merely enough to relieve tension on the upper perch in order to remove the above mount.
This picture is of the spring off but shows how much you need to compress each spring. Again these springs are odd most have 3 coils exposed on other cars ive gotten the clamps on 3 coils on one side and 2 the other at 180 deg apart not the case here maybe 150-160 deg and 2 coils a piece. I clamped down one side enough to fit the compressors over three coils on the other side.
3. Now is the point at which you need pass thru sockets if you do not have them you will need to devise a way to secure the strut rod from spinning while you losen the the 17mm. Ive read using strap wrenches and other methods but I dont want my finger in between a compressed suspension spring for any amount of time

Pass thru sockets work the BEST for this hands down

Now you may get the urge to be uber brute force and double fist this bad boy:

but save your arms!
Position your hex socket or key such that it rests on the work surface, use a combo of extensions or one long extensions and insert it through the lower strut bolts hole to prevent the entire assembly from rotating, then use your body weight to apply pressure on the 17mm pass thru ratchet.
Once the 17mm nut is off you can disassemble by taking off the strut mount, conical washer, upper perch, compressed spring, dust boot and bump stop.
Now I took this time to clean up my upper perches. Water and soap with a brush and viola!
Here is a comparison of the H- Tech vs Stocker
Reassembly
1. Place the bump stop onto the strut rod, then the dust boot over it. Then place the spring compressors on your new springs and compress it down. I would place it on the strut after a few turns to gauge how much I needed to compress the spring. I didnt want to go too much b/c I didnt want too much slack when trying to align the upper perch later.

2. Next place the upper perch on top of the compressed spring, followed by the conical washer (BIG SIDE DOWN) and the mount itself. Dont worry about aligning things yet, just get it loosely assembled.
3. Now you have a loosely assembly strut assembly (phew

) Make sure the lower coil rests in the notch of the bottom perch, when correct the "Tein" and part number will be facing the outside of the vehicle. Next back off the tension on the spring compressors until the spring starts to interact with the upper perch.
I apologize for not having a picture of this step
4. Now the upper perches are angled and have a recessed rubber bottom. The top coil of the spring fits over this piece. Before releasing too much tension orient the perch so that the two drain holes will face the the outside of the vehicle. Also the perch has two rubber nipples that should be relatively parallel with the wheel. Dont worry about the top mount as it spins freely when fully assembled and is no dependent on any specific orientation
5. Hold the perch steady and back off the tension fully.
6. Tighten the top mount nut down but not torqued.
Notice in the finished photo the perches angle upward and outward, that is the side that faces the outside of the vehicle. In addition to that the ONLY side specific part is the strut ALL other parts are not side specific. This is confirmed by the part numbers. Now to be sure your finished assemblies go on the correct
simply check the part number stamped on the lower strut body 339096 is right 339097 is left.
Repeat for the other strut assembly.
Now to the rears:
Tools:
3/4" (19mm) socket
6mm hex key
1/2 and 3/8 inch drive ratchets
Spring compressors
17mm wrench
**Note**
I dont have a vice nor workbench to clamp the assembly into so I use my kitchen table and an extension to prevent the shock body from turning while tightening things up/compress-decompressing springs
Compress your springs by placing the compressors on each side of the spring (180 degrees apart) and alternate the coils clamped. Also make sure not to use the bottom coil as it will result in the spring resting against it when decompressed and you wont be able to pull it out.
Good:

Bad:
Alternate tightening the clamps to evenly compress the coil. Once finished place the spring on the shock.
**Note the tightly wound coils are the top while the not so tight coils are the bottom. Also the bottom coil fits right into the notch on the lower spring perch use this as a guide to keep the mount, rubber and coil aligned when decompressing the spring.****
Then place the spring rubber (that what I call it maybe b/c im a NASCAR fan

) and mount atop the coil.
Use the supplied lock nut and add a washer to the mix (my KYB's only came with the lock nut no washer

).Hand tighten. Now this is the part where a vice and bench will make your life much easier but if you are like me or assembling in the parking lot use a 3/8 extension or bolt and place the whole assembly such that the shock body wont turn.
I placed it in the tray that the compressors came it then used my 6mm hex socket and a 17mm wrench. The trick is to hold the nut with the wrench and turn the hex (the shaft) inside the nut to tighten.
The mout and spring rubber will be tilted this is fine once the nut is tight the mount will be level.
Once the mount is tight back the tension off but not all the way as you need a little play in order to align the two body stud correctly. This doesnt need to be 100% perfect but you can get it VERY close. I used the extension to represent the axis of the control arm bolt. The two studs be perpendicular to the bolt head.
Once aligned back the tension all the way off making sure the lower coil rest at or near the lower spring perch notch.
Done with 1/4" saggy butt shims on top!