Framing Up: Ideas on How to Finish Shaft with Beaded Paneling

Headers For The Ceiling

Measure the inside dimensions of the rafters and cut two 2x's that are the same width as the ceiling joists, to that length. Slide them in place and nail through the joists into the end of the headers. Be sure to keep them on your plumb lines.
Just like the jack rafters up top, if you light is smaller than the opening between joists, you'll have to scab on some 2x material to close the gap. These are just face nailed into the joists. I hope for your sake that the joists are not 100 year old oak like that in some older houses I've worked in. They are very hard to nail into.

Framing Out The Light Shaft

How you plan to finish the shaft is something you need to think about. Some people put light fixtures recessed into the shaft walls for use at night. If you're going to finish it with beaded paneling, you have to frame it to facilitate that fact. Those types of things are special considerations. What I'm going to go through is just the framing needed to install and finish drywall.
Unless you did the 'straight out' method, you'll probably have some angled cuts to make. You basically need to measure from the bottom of the top header (in the rafter) to the top of the header directly below it (in the ceiling). Measure each corner in this fashion and you need to cut 2x4 studs to those measurements, considering the angles you'll probably need to cut. You want two studs at each corner to provide stability and a nailer for the drywall. Then you'll want studs between the corners, 16" o.c. (on center). That means from the left of one stud to the left of the next is 16", not 16" between the studs!
Use your level to make sure you are measuring accurately. Measure out your 16" centers on the ceiling joists, all the way around the opening. Then use your level or plumb bob to get a plumb mark on the rafters and headers above. Then measure from each mark on the bottom, to its corresponding mark above. Again, note the angle.
Toenail the 2x4's into the framing with smaller nails, like 8d (8 penny). Having a helper in the attic while you stand on a ladder nailing the bottom is really helpful. Pick somebody you trust though, you don't want them dropping a hammer on your head.