Measure the existing doorway space. Then cut your timber to size for the frame. Lay the timber piece out on the floor and nail the frame together. Stand the frame up, position it inside the doorway and nail it in place. In this case, because this doorway is over 800mm, we placed an extra stud in the middle to stop the plasterboard warping. Put some packer timber around the edges of the new frame so the new plasterboard will sit flush with the old plaster.