The equilateral triangle
The first rule is to place your monitors in such a way as to create an equilateral triangle (60° angles with equal distance between your ears and the left and right monitors). The distance between you and the monitors is dependant on the monitor and its size. The monitor manufacturer will usually indicate the distance for optimized listening in the user manual. The bigger the monitor, the longer the distance. This should be taken into consideration when purchasing. Sometimes bigger monitors will not be the best solution.
Up in front
Monitor height should also be considered. The object is to get the tweeters lined up with your ears. If, due to your specific home studio set up, this is not possible, you could tilt the speakers down a bit (if the speakers are too low, place them on a support) so that the tweeters are directly lined yup with your ear level. Monitors should always be directed to the ear direction and not towards the wall behind you.
Walls and corners
Within your four walls, you must keep a symmetry. When a sound source is placed near a wall, or worse, in a corner, certain bass frequencies are acoustically amplified, which is to be avoided at all costs since it falsifies the spectrum.
Stand ‘em up
You don’t want resonance in a home studio. It might be very difficult to eliminate acoustic resonance in your home studio (this usually involves advanced acoustic technical knowledge) but it is a bit easier to decrease the mechanical resonances by “detaching” the monitors from your desk. A few solutions are possible: foam mats under your monitors (that also act as risers) or better yet, monitor stands. Resonance from your monitors to your desk will be greatly decreased and you gain some space. You can combine the two solutions by placing the foam mats on the stand and then the monitor on top of that.
Your monitors are installed, just plug them in and adjust to preference!