I would suggest you only take one window out at a time you may need to get measurements from another the same size. I would remove a screw on each window and check it's threads. It there aren't any left you will need to do all the
windows because the wood is so bad it wouldn't hold a new screw at all. My '76
Trillium was in that condition. You will need access to a table saw to make the new wood pieces. Most everything else just needs hand tools. The edge of the wood is beveled at 45 degrees on the outside edge all around. Use a putty knife to peel back the ensolite covering to gain access. I used spray contact cement to re-install it on mine. Good luck! Go slow so you don't damage anything.