Dimensions and some important measurements
Actual lumber sizes vary from place to place so use all dimensions as a guide only.

The important measurements are....
1.) The overall height and width of the door frame should be 10mm (3/8") less than the overall height and width of the rough opening in the wall, to allow for some play.

2.) Allow enough gap between the door and frame to ensure ease of opening. In this case at least 5mm (3/16") clearance all the way around the door.

How wide should the door frame be?
Usually the width of a door frame should be the same as the thickness of the wall, including the interior lining and the exterior siding. This is so the frame will be flush with both the inside and outside of the wall allowing for trims and moldings to fit neatly around both the inside and outside of the door frame. However in this case, the frame is wider than the wall which means it will project into the building an inch or so, reflecting our personal preference regarding the finished look.

Instructions

Step 1. Rebate the frame
Cut a rebate in a piece of lumber 150x50 x 3100 long (2x6 x 124" long). This is best done with a bench saw by making two cuts along the length of the lumber as follows: make one cut 50mm (2") deep along the edge and one cut 20mm (3/4") deep along the face. See the drawing.

Step 2. Cut the frame members
Cut the 3 frame members to length. The frame members comprise of a head piece and two side jamb pieces. Cut the head 640mm (25 5/8") long with a 45° angle cut at each end.
Cut the two side jambs 1200mm (48") long with a 45° angle cut at the top ends and a standard square cut at the bottom ends.

Step 3. Glue and fix
Glue and nail the frame members together.
Nail a temporary spacer to the bottom of the door frame to keep the side jambs parallel.