Most interior doors (excluding bi-fold doors) are pre-hung assembled or knock down ready to assemble.
To accommodate a pre-hung door the R.S.O. (rough stud opening) must be installed to allow for shimming tolerance, this is required for both sides and the top. The rule of thumb is one inch wider than the door width and one half inch higher including the door frame. This is to allow for any shrinkage or twisting in the rough stud opening. If your door is the knock down ready to assemble type put the unit together following the instructions. You will find that you can assemble for either right hand or left hand hinge, swinging in or out of the opening.
Apply a piece of 1x2 across the door at the bottom (right jamb to left jamb) to hold the door in the front plumb and square. Set the door in the opening and using a carpenters square and level shim with cedar door shims and fas-ten with nails. Do the hinge side first then the others shimming where required top and side.