First, decide which acoustic response model you want to use. Once decided, you can start to treat the room accordingly, depending on the concept chosen:
Originally Posted by Jens Eklund
1. Learn how to make measurements: REW - Room EQ Wizard Home Page
Don’t do anything without measurements.
2. Find the best position. Usually centered up against a short wall, is the best place to start*. Confirm with measurements. If the room is big, you can experiment with a position away from the wall but then usually more than approx. 1,5-2 meters from it (speakers).
3. Identify and treat your modal and SBIR
related issues and educate yourself about different bass-absorbing techniques (pressure based vs. velocity based etc.).
4. Treat areas that otherwise causes early reflections.
5. If the room is big enough, add diffusers (but read up on how to use diffusers before going nuts).
Always base your decisions regarding different treatment, on measurements. Avoid thin (less than about 4") porous only absorbers (including wall to wall –carpet, drapes etc.) unless a measurement indicates the need for it. There are many informative threads lying around; find them.
* But not always: Mixing Room Layout...Help me make it better!
And before posting your graphs, check out this post: Before posting your measurement results