Determine the height you want to make your door. Add an extra 7/8" to account for the reveal around the top (3/8") and bottom of the door (1/2"). Measure up from the baseboard along one of the inner door frame boards to that measurement and make a mark. Repeat for the opposite side as well.
Cut a 2" x 4" board to fit in-between with the bottom lined up with your marks. Make sure your header is level using a level and adjust as needed. Secure your header using 2.5" wood screws by screwing to your posts. Cut another 2" x 4" to fit directly above your header to form an L as a header. This should be flush with the outside of the lower header piece. Secure this piece with the 2.5" wood screws as well.
Now you want to install posts on the sides to form an L similar to how you did the header. These provide areas to secure your wiggle wire lock channel later on. The easiest way to measure these side posts is to place the bottom on the ground and just mark underneath the hoop and cut. Secure these additional side posts with the same 2.5" wood screws.