GIK and Acoustimac are two good room treatment companies. Both offer advise on how much you should get and where to place them (but of course they want you to spend more).
Or, go to Lowe’s or Home Depot, buy some Roxul Safe N’ Sound, put it in a wooden frame (that you also buy there), then wrap it in a breathable fabric (go to a crafts store).
Quick and dirty (and wrong Roxul product):
https://youtu.be/r6gPmWtXdlEMore in-depth and cleaner:
https://youtu.be/U8C4mcleE5UFirst reflections are the main problems. You can calculate it mathematically, but easiest way it to get a non-magnified band mirror, sit down in the seating position and have someone else run the mirror along the side walls until you can see the tweeter/woofers in the reflection, that’s your first reflection points.