This is the only step you'll take from inside your home or attic. Frame the opening by doubling the rafters. Nail a doubleheader and sill to the top and bottom of the in the respective location. Drive a screw through the roof at each corner of your opening to serve as a marker for the outside of the roof.
Note: This is strictly a two-person job. The person on the roof must be on it securely. Use best safety practices to avoid falls or injuries. Have the other person inside the attic or home, ready to receive the cutout as you saw through the opening.