serpentine belts

their should be a sticker with diagram of the belt replacement under hood.

Yes, you may also wish to purchase a Haynes manual for your vehicle as sometimes you need to access the belts from a wheel well or other location that is not immediately obvious. Belt tensioning devices from manufacturers differ widely so it would be good to know how to properly loosen and tension your belts before beginning.

loosen the alternator and the a/c tentioner

Bingo. Changing the A/C and power steering/alternator belts on a Subaru is ridiculously easy. Just remember to loosen the bolt holding the tensioners in place before backing them off. After that, just slip the belts onto the pulleys, tighten, replace the belt cover(s), and go drive!

when you are replacing the serpentine belt, you should see if you need to replace the timing belt as well. this is because you have what is called a interference engine. this means that if the timing belt brakes your piston will hit the valves causing a lot of damage and costing a lot of money. if you look in your owners manual it should say when it needs to be replaced.

That would require taking off the timing belt cover, which is a little bit harder. Just wait for the 90,000 mile service interval to replace the timing belt.