Firstly wHAt type of door, a solid pine one, or a pressed hardboard one(hollow), if hollow tap the door at both sides in the middle edge, to find the lock block, this is a block of wood put into the door at manufacture to fix the latch or lock and handles onto, sometimes marked on the top of the door, but always check, now put a piece of tape on the opposite side of the lock block to signify the hinge side, place door against frame,try to visualise the amount of timber you need to take off both sides and top and bottom , if the frame slopesor the floor slopes, they do in a lot of homes.
Use an electric plane, slowly and with about a half mm cut, remember when you put the planner down the cutters will still be moving very fast, always pick the planner up by the handle never the cable, or by putting hand underneath, they will take your fingers off.
When the door is a goodfit, mark the hinges, remeber the hollow doors only have a thin frame inside, so dont hit the chisel to hard as you may well dislodge the frame as its only glued.
For hollow doors the frame is secured together with huge staples, that knacker planner blades, when you hit them, so have a spare set of blades, and ALWAYS DISCONNECT THE POWER PLUG WHEN CHANGING BLADES.