As funfool mentioned, examine the current door to decide if any adjustments are needed on the new door to fit better int the opening. I always pop the door hinge pins and with a pen use half of the hinge as my template to scribe onto the side of the blank door for mortising purposes. when looking down on the mortised section go from left to right with the router. when determining how far from the edge of the door to put the hinge look at the hinge as it sits in the door opening and how far it is placed from the door stop. figure a fat 1/16" for the door to be away from the stop on the hinge side so that it clears the stop when swung. If using new door hinges with the new door get the hinges placed on the jamb first and then pop out the hinge pins and mount half of the hinge to the door while leaving the other half on the jamb. make sure the jamb hinge half is snug and pull all hinge spacing measurements from the top hinge. after you have the hinge spacing measurements and the hinge halves morticed to the slab, loosen the hinge screws just a tiny bit to help when putting the door in the opening and lining the two hinge halves up. once the two hinges are together put the pin back into each and tighten up the screws and test the fit.
working with a pre existing opening can present challenges too because of how it is installed etc.. so examine everything closely and work out all square, plumb, level surfaces with a level- 4' and 2' levels help to find how square the opening really is. check to make sure the door stop is straight as well and how level the floor is itself out way from the door in the amount of space that the door requires to swing away from the opening. sometimes an out of level floor will not allow a door to swing to its full potential...
drill the 2 1/8" bore hole after the door is hung and you can line up the bore with the strike plate already in the existing jamb
another tip from someone who has battled many out of square openings.. try and find one square/plumb/level side in the opening and work everything off of that side, if none of the sides are square/plumb/level then do the best you can.
Last edited by hand drive; 11-13-2012 at 10:06 AM.