1) This is very easy with proper preparation, will save you a lot of money, and will give extra years to your 5EAT.
2) I don't have pictures of it being done on an LGT, so I'm blatantly using other peoples' photos and videos to illustrate the point.
I do this to all my cars, same procedure. All my cars shift perfectly.
Things you need:
1) ~12 quarts ATF (OEM Subaru HP+; Amsoil ATF; Mobil 1 Full Synthetic ATF)
2) Funnel that fits into your transmission's dipstick tube / fill tube.
3) Graduated bucket (bucket to catch fluid, with markings to tell you how many gallons are in it.
4) Plyers, basic other hand tools.
5) Optional part 1 of 2: 3/8" doubled sided hose barb, auto parts store or hardware store. You might not need this, but it costs $2.
6) Optional part 2 of 2: 10' of 3/8" I.D. hose, cheap, auto parts store or hardware store.
5) The cognizance that cars are cramped, dirty, sensitive machines that you should not mess with if you do not know what you are doing.
Summary of Process:
1) Unplug ATF cooler line return hose from any return pipe section. (If you don't know whether a line is output or return, unplug a line and run the engine for a couple seconds. The line that spits out fluid is the line you want.
2) Optional: Connect an extension hose (use barb adapter if attaching hose to hose.
3) Put graduated bucket next to open driver door, put extension length of hose in bucket. Maybe tape it so it doesn't fall out.
4) Turn engine on. Turn engine off when A) bucket fills with 3-4 quarts of ATF, or 2) trans starts making an odd noise.
5) Use funnel, put matching amount of fresh ATF into trans.
6) Repeat 4 and 5, this time quickly switching into Reverse, Neutral, Drive (but don't let the car move).
7) Repeat 4, 5, 6 until you run out of fresh ATF, or ATF starts coming out clean.
8) After car is put back together, drive for 10 minutes around town (with funnel and extra fluid in car). Then with car idling, check fluid level, get it between the two HOT marks.
Note: People like to complicate the heck out of this. Keep it as simple as possible, as few steps as possible. Example: If you have a friend to start and stop the engine at your command, then you may not need to rig up an extension hose if you can get your fluid catch bucket under the car. You also don't need a graduated bucket if you have a general sense for how much fluid is going in your bucket. Absolutely no need to pull the pan plug. Waste of time.
Watch this video
Skip to 2:57 for instructions. Everything else is a waste of time.
This great walkthrough
shows the several trans ATF cooler hoses/pipes you can unplug on page 3,4,5, and it shows a sample double sided 3/8" hose barb on page 1. http://subaru.rockhopjohn.com/Transcool.pdf
Example on a Toyota Truck (skip to Step 4 and 5): http://www.toyotanation.com/forum/61...ion-flush.html
On the LGT: The cooler pipes run along the driver side framerail up to the front of the car. You can pull a line under the car if you remove the skid plate. There are lots of pictures somewhere on this forum of this area of ATF cooler lines. If you have links, post them please
Not all ATF cooler line routing is the same, from Legacy USD to Legacy JDM to Outback to Tribeca, 3.0r, there are variations. So pictures may or may not be indicative of what you will find on your vehicle. For example, the Tribeca ATF flows through the radiator in the opposite direction as the OBXT / 3.0R / LGT. So take the extra minute to check for yourself, assume nothing.
Basically, if you can't find the cooler lines, or can't get to them, that is a good indication that you might want to seek the guidance of a friend, or just simply not do this yourself. It's the kind of thing that is easy to do, and easy to screw up such that it ruins your day. As some CarTalk callers have admitted, even a simple oil change can end up taking days and require a tow truck if you unintentionally make a mistake.
Ask questions if you have any.
A word of caution...
A few days ago, someone on these boards reminded me of a risk of doing a complete flush all at once: If the fluid is VERY old, then there could be a lot of gunk buildup in the transmission.
ATF contains detergents, but this detergent cleaning ability fades over time. Putting all new ATF in a gunky transmission could dislodge all that gunk too quickly, causing the clogging of certain parts of the trans.
If you want to be careful to avoid this risk, then BEFORE you do the full flush:
Remove one old quart of ATF, and replace with one new quart, then drive around for 100 miles. Then repeat once or twice more, or until you feel confident that you are not going to unleash a wave of gunk deposits. This is being seriously careful; use your judgement.
I would not worry about this problem at 30k or even at 60k miles, but at over 100k, consider it.