If you can cut a 2x4 and drive a nail, then you have all the skills you need to frame a rough opening for a door. As with most homebuilding tasks, however, there's a fine line between getting it done and doing it efficiently, without causing headaches down the line.
Whether framing a rough opening in a new home or on a remodeling project, I follow the same sequence. I always frame rough openings before I install the surrounding studs. This gives me plenty of room to nail the trimmers to the king studs and the king studs to the header. If the wall is load-bearing, I refer to the plans for the proper header size. If not, a 2x4 on the flat replaces the header. In this case, the plans called for a double 2x8 header. I added a 1/2-in. piece of plywood between the 2x8s so that the header width matches the 2x4 stud thickness.