Put in very simple (and mostly wrong) terms, signals below 250Hz behave like roudy school-children. They tend to lurk in the corners of rooms, and then get disruptive when the music starts. Trap them with stuff not that far removed from loft insulation in the places where they congregate.
Above 250Hz, your room becomes a big pool table of sound. Sounds reflect off the walls like a pool ball bounces off the cushions of a pool table, By putting absorption and diffusion products in places where the sound is likely to bounce back and hit you, you get to hear more of the loudspeaker and less of the room joining in. The obvious places are behind each speaker and behind your head.
Sound also reflects off the side walls, floor and ceiling. I'm a dissenter about the importance of treating these (first) reflections, as I think this is what separates exciting and overdamped sound, and too much first reflection kills stage width. But, if you want to try, sit in your seat with the speakers installed and have a friend move a mirror flat along the side wall. The point where you can see the tweeters reflected in the mirror from your seat is the first reflection point and this can benefit from absorption (something like the loft insulation again) or diffusion (something random like a bookshelf or a dedicated diffusor).
Finally, between the point where you treat the bass and front/rear reflections, and the first reflection, experiment with positioning your listening chair. The 38% rule applies here, but I'd be more experimental. Sit the chair roughly 4/10ths the way along the centreline of the room (either close to the speakers or at the other end of the room) and listen, reposition slightly, listen again. Keep going until happy. If you are using electrostatics, try again, this time with the speakers one-third of the way along the length of the room, the chair one third of the length of the room from the speakers.
Ultimately, a better result can be had by measuring the room and treating its specific problems. There is a problem with this; in the process of getting your room ruler flat, you dial out any form of good sound in the process. Measurement good, measurement and pragmatism better.