The phrase "they don't build them like they used to" comes to mind when your renovating an older house like Jess and I. That is especially true when it comes to doors. The doors that were hanging in the house are solid all the way through... this is not common now a days... there are options out there but this luxury comes with a premium... one that Jess and I are not willing to shell out. So we basically took the doors that we have and did a simple sanding job to take the imperfections out then we painted them with a semi-gloss latex paint and we were done.
For most of the house we still had the old frames up so hanging was not that big of a deal... basically everything was the width and height that it need to be... you just had to find the exact door that was associated with its old frame. A simple phillips head drill bit with a driver and new hinges and you were ready to go.
Now part of our renovation to this house was moving walls to open up the living space and when you do that you are going to have new studs, new openings, and new door frames. The only thing you need to remember about rough opening for doors is this: what ever the width of your door needs to be add 3" to that and you will have your rough opening (width). Whatever door height you want just add 1.5" to that and you will have your rough opening height.The logic behind those numbers is that you door frame material is usually around 3/4" and then you need another 3/4" tolerance on each side to make sure you door frame is square... at the jamb that gives you 3" and at the head of the door that gives you 1.5".
Once you have your rough opening you need to construct your frame. You can go to any hardware store and they will usually have pre-made door frames for both 2x4 and 2x6 walls. If you have any 1x materials laying around you can always make you own frames by ripping them down to the correct size just remember to consider the dimension of the gyp (also referred to as sheet rock or dry wall) when you are figuring your wall width assembly.
When attaching your door frame consider where you will be putting the screws to hold up the frame. I usually locate my screws about 2.5" from the door swing side and then place my 1/2" x 1 1/2" door stop over the screws so I will not need to use wood putty to cover my screws. Once you have everything square just use spacers to fill the gap between your studs and your frame... use a 3" screw to make sure you have a great connection... put in the door stop... then your door is ready to install.
For the door frames that we didn't have to make ourselves, I just had to use the planer to shave the bottom, top, or side of the door in order to make it close smoothly.
and there you have it!