DIY; I recommend getting 20L of ATF for it. Takes about a 1-1.5 hours to do. Needs two people realistically.
You'll also need to be able to access the trans drain plug (needs an 8mm hex head/Allen key to undo it), and to access the lines / hoses for ATF near the radiator
- Drain the oil pan and fill tranny with fresh fluid
- Disconnect the hose from the return hard line, the rear of the two near the water pump/radiator hose:

- Attach some ⅜" (10mm) hose to the hardline and plug the end, this will stop too much air being sucked into the system.
- Feed the return hose into a bucket, preferably white with graduated marks so you know how much is being pumped out.
- Get about 30-40 cm of ⅜" (10mm) hose and shave the outside off one end tapering it

Use this to extend your ATF filler. Then shove a small funnel in the other end. Alternatively, just find one of those flexible ones with a long spout that fits into the ATF filler.

- Replace the drained fluid with the same amount of fresh
- Start the car and let idle with one person filling as fast as possible (it's not very fast) and the other monitoring what is coming out of the of the tranny; you won't be able to leave the car on for more than a minute or two each time as the fluid is pumped out much faster than you can replace it; this is where the bucket with marks for volume on it comes in handy; so you know how much you've drained and how much needs replacing

- Continue this process until you start to see clean, fresh ATF being flushed through; if it's anything like the few times I've done this, it'll be bloody obvious
- Once fresh fluid is coming out, replace the removed return hose and fill the trans to appropriate level.
Depending on how bad it is, and how much fluid you have available, you can also cycle through gears (foot slammed on brake!) to help circulate the fluid a bit. I usually do this when the fluid is starting to become more translucent.
All up, if you have a way of raising the car, or getting underneath it, and can spare the time, it should cost about $200. I get Penrite ATF-FS in 20L drum from Autobarn for $189. Then another $10-20 in random bits (funnel, bucket, etc.). Works out a lot cheaper than any place in Brisbane I've found that will do a similar thing (about half of most workshops and about a third the price of the only Subaru dealership that will do it up here). I spoke with the foreman at City Subaru in Brisbane and he says while the power flush done by a workshop is better, this method will get "FKN close to a 100% flush".
HTH
Sean..