Fairly simple: just level the trailer, presuming it is on level ground, then measure the distance from the coupler to the ground. This is the height the hitch ball should be set to. Then if you have a weight distrubution hitch tighten the weight bars to bring it back to level. If the hitch is just a ball mount then put the coupler on the ball, measure the amount of sag, add that to the ball height and set the ball height to that dimension. The goal is to have the trailer level when it is being towed.