The first thing to do is check the width of the framed opening. Normally the width of the opening is 2" bigger than the door give or take a little either way.
After applying the casing carefully stand the door back up and it is ready. I have found with my experience that this is an easier method of installing interior doors for beginners to learn as opposed to hanging the door without the casing. If you are working where there is going to be future carpet inside and outside the room where you are installing the door you can hold the hinge side of the door up about a half inch off the floor with a small half inch thick block. In other words, set a small 1/2" thick block on the floor next to the stud on the hinge side of the door and when you have the door ready, set the hinge side door jamb on top of this block. This way when the door swings into the room it will not rub on the carpet. Assuming there is going to be vinyl or linolium or some other thin layered flooring either inside or outside of the rooms where you are installing interior doors, it is done a little different. You want to set the jamb of the doors down to the floor or as close as possible. Hopefully the floor is nice and level or you are in for some extra work. You have your 1/2" little block on the floor for the hinged side jamb of the door to sit on. Just before sliding the door into the opening unattach the door from the jamb on the knob side if it is still attached. From the inside of the room set your door into the opening making sure it is pushed completely into the opening so the casing is tight to the sheetrock all the way around the door. Slide the hinge side of the door over tight against the rough opening on the hinge side. Take your big finish nail gun and shoot a nail into the outside edge of the casing about 2" above the bottom of the casing or 2 and 1/2"up from the floor. Take your 4' wood level and put it against the bottom two hinges opposite side of the casing and lift the knob side of the door up until your level reads plumb. At this point you can shim the knob side up so the door stays plumb. Make sure while doing all of this that the door stays pushed tightly into the rough opening. Door is plumb now at the bottom two hinges shoot a nail on the outside part of the casing directly across from the bottom hinge. Making sure again the door is plumb and pushed in tight, shoot a nail in across from the middle hinge. Move your level up and check for plumb from the middle hinge to the top hinge adjust as needed and shoot a nail in about 4 to 6 inches down from the top of the casing. Now with the door still shut shoot a nail in the top piece of casing that runs across the top of the door on the knob side now also in the middle and back on the hinge side always being sure to be shooting the nails in on the outside or thick part of the casing with the big gun. Open the door, standing directly in the middle of the door facing the jamb on the hinge side shoot a nail in the jamb about 1" above the top hinge move your gun directly across over the stop and shoot a nail in the jamb on the outside of the stop. Repeat this at each hinge then again a couple inches above the floor. Close the door, make sure the crack or opening on the knob side between the door and the jamb is the same from bottom to top and preferably nice and tight without the door hanging up or sticking. Shoot 4 nails in from top to bottom on the outside of the casing spreading them out fairly even. Now grab your shims and on the outside of the door about 2 to 3"down from the corner on the knob side snug the shims into the gap between the stud and the jamb carefully, then shoot 2 nails through the jamb through the shims into the stud. Repeat this 3 more times ending up about 2 to 3" off the floor. Your interior door is hung. Not done yet though one last part to installing interior doors and that is to case the outside. See casing interior doors for the total completion of this door.