There are some guidelines you can follow.

Put bass traps in corners because thats where bass 'gathers'. - if you have foam bass traps you can increase their effectiveness by placing them off the wall a bit.

Bass is omni-directional so it is worth treating the area behind the speakers, but mid and high frequency is not, so you will get more effect from thin absorptive and/or reflective panels if they are placed 'in the line of sight' of the speakers.

Put absorption or diffusion at first reflection points. ââ'¬â€œ Use a mirror and a helpful hand to find them.

Diffusion works well on the ceiling and maybe the back wall if you are a reasonable distance from it.

If you use membrane bass traps you will probably also want some broadband absorption in or across corners or anywhere that sounds 'coloured' that is 'in sight' of the speakers. Foam bass traps are basically broadband absorption so if you use these in all corners you probably won't need extra broadband treatment as well.

Remember there are not only corners where the walls meet but also where the floor and ceiling meet the walls.