Replacing the missing glass in my first frame was the step I was dreading the most because it was so far out of my scope of experience. I dug around online and found several fairly helpful sites with directions (like this one), and since I was not replacing glass in a window that would be used as a regular window, I chose to skip using a glazing compound.
Flat head screwdriver
The glass was already fully missing, so I did not have to worry about carefully removing it as directed on the different websites. However, one of the pieces of wood that would have been holding the glass in place was loose before I began cleaning the frame, and it separated from the rest of the frame during the cleaning process (it also cracked, but was thankfully easy to glue back together).
At first I thought I would be able to slide the replacement glass in between the remaining three wood slats and the frame, but they were solidly in place and there was no wiggle room for the pane. I had to very carefully pry the last three wood pieces off the frame using a flat head screwdriver. I broke the rusted nails off of the pieces using pliers. Thankfully, they broke smoothly, but you may need to file the ends if there are nails in your frame.
I picked up an inexpensive box of glazier points at Michaels and followed the very simple directions on the back of the box for how to arrange and install the points.
Once the glass was held in place with enough glazier points, I replaced the wood, using wood glue and C-clamps.
This is the end result:
Coming up Next: Staining and Finishing the Frame