Basically, and this is like, a simplified version.
Break up reflections to the left and right of you sweet spot by placing scatter tiles on the walls, scatter tiles to the rear also, centering on head height. Monitors should be reasonably separated from the back wall.
Bass traps in the areas where walls are adjoining, and in the corners are very important in killing standing waves and exponential bass buildup.
If you can wait a bit, I`m gonna be producing my own accoustic tiles, at at least half the price of any other producer, so I`ll be able to provide you with cheaper bass traps.