Framing a door is a relatively easy task. For this you will need the following items:
Know the rough size of your door
If the door you are installing is 36 wide x 68 heigh, then your door is a 3068. The rough opening (RO) of a door will depend on how the actual frames were made; for example, a wood frame will be 1 thick, a hollow metal steel frame is 2 thick, a timely frame will have ½ thickness and etc. You may also try to contact your manufacturer if the door is custom built.
If a door frame thickness is 1 then your overall rough opening width will be 2 above the door size and your height will be 1 above door.
If you have a hollow metal frame (HM) then the overall size will be 4 on width and 2 on height.
Know Your opening Size
Your door opening size (DO) will be the RO + Ύ or 7/8. For example, your RO is 38 x 97, this means your actual opening is 38 Ύ x 97 Ύ. Dont make your opening more than 1 because you will end up using too many shims to hold the door, and your frame will not be as secure as you want it.
Frame your opening
Once you need your RO you will start by placing the bottom plate on the floor, marking both ends of the trimmers (see the video for parts) and the king studs. Start building the opening by nailing the king studs to the bottom plate, nail the trimmers to the king studs. Note: do not nail the trimmer to the bottom plates and the header or you wont be able to adjust the opening! And finally, nail the header to the king studs. Once you have completed this, you can install the cripples and the top plates.
Remember to contact your manufacturer to determine the RO of a door. This is important because you may not have the doors until the building is framed, and you do not want to be framing the doors twice.
Do not nail the trimmers to the bottom plate or the header. The trimmer should only have (6) nails. Place them in pairs close to the edge 2 on the bottom, 2 on the middle and 2 on the top, this is done so you can use this trim to plumb your door once is installed.
Your header will always be 3 more than your RO (rough opening).
Make sure to not place any sheetrock screws in the trimmers. Once again, you need to have these trimmers free in case you need to plumb the door.