First reflection points are direct waves reflected from the walls of your room, sides, front, back, ceiling and floor. They must be treated to avoid the coloration created by the room, with absorption and diffusion.
On these points you have to place absorbers and diffusers, and how to locate these points?
You can use a small mirror or a cd.
An example: At your ear height, stick it in the left wall or with someone to help you move the mirror left-right, up-down until you see the tweeter of your right speaker, of course, you must be sitting on the sweet spot.
In those points where you see the tweeter, you have to place the panels.
Typically positions of absorbers and diffusers for first reflection points on rectangular studios.
Absorbers: Left and right wall, left and right ceiling, back wall.
Diffusers: Front wall, back wall, rear sidewalls, ceiling. Recommended for distances of 2m or greater.
Personally, I like using absorbers on the left and right walls and ceiling.
Diffusers on the front, back wall, and rear sidewalls.
I use RPG absorbors 5cm with an air gap of 5cm. GIK Versifusor in front, back and rear sidewalls, RPG Skylines in my back wall.
This makes an open sound with better depth and soundstage. I don´t like too dead control rooms. I prefer a controlled accurate sound with a spark of musical feeling.
Remember we are talking about first reflection points only not bass traps of the room.
Now will see the placement of the monitors, another important factor.