If the opening was framed by someone else, you may want to break out your level and framing square and check this also. Drywallers sometimes believe the rough opening was meant for them and will let the drywall run into the opening. If this is the case use a drywall saw or sawzall to cut it back.
Once all the vitals have been checked your ready to hang a door. Prehung doors come assembled a couple different ways. They can be bought with trim already mitered and nailed on to one side and without trim. If there is no trim installed, I like to put it on before I put the door in the opening. The trim is installed on the hinge side.
Most doors open into a room and against a wall. When putting the Prehung Door Guys opening, try to put the door in the center of the opening. The door jamb should be able to move to the left and right in the opening. The gap between the door and jamb on the hinge side is usually about an 1/8" of an inch. This dictates the gap or space you should have all around the door. Move the door jamb to the left or right until you have that same space at the top.
You then nail the trim on the top hinge side and the bottom hinge side. Then nail the strike side on the top making sure you still have an equal space. Nail off the rest of the hinge side with 3 or 4 more nails. The 2 nails already in the top are all I usually put in. Now nail the rest of the strike side starting at the top and working your way down, maintaining the same space as the top and hinge side.