1) Build the rough opening in the wall as follows:

• Height: Nominal door opening height + 1 1/4" (31.8 mm).
  Example: 84" + 1 1/4"= 85 1/4" (2133 mm + 31.8 mm = 2164.8 mm).
  
  
• Width:  Nominal door opening width  + 2" (50.8 mm).
  
• Example: 36" + 2" = 38"  (914 mm + 50.8 mm = 964.8 mm).
  
  
• The rough opening should be square and level.

2) Lay down one jamb against one side of the rough opening.

3) Assemble the horizontal header in the vertical jamb.

4) Slide both components to overlap the wall by approximately 1" (25.4 mm).

5) Assemble the second vertical jamb into the horizontal header. Slide the jamb to overlap the wall by approximately 1" (25.4 mm).

6) Complete the  assembly between the vertical jambs and the horizontal header by bending the tabs or fastening the screws at the top of the horizontal header.

7) Fit the top of the frame between the wall studs by tightening the compression anchor at the top of each jamb.

8) Adjust the frame to be square, level, and plumb, with jambs in the same vertical plane.

9) Anchor the frame to the stud at the bottom of each jamb with the drywall strap or the screw base anchor (screws by others).

10) Anchor the frame to the stud at the center location with the drywall strap (must be installed on fire rated frames).

11) Verify the height and the width of the door opening (tolerance of 1/32" (0.8 mm)). The frame should be square, level, and plumb, with jambs in the same vertical plane.