Hey there,
Here is what I did, it kinda depends on the film holder you are using but this should get you started in the right direction. If you have a lens board w/o a lens, mount that on the front of a view camera and and put your 'donor film holder' in the camera back.
Then I slid a piece of decent size dowel rod throug the lens board opening until it landed squarely on the film holder with the dark slide removed. Mark it with a fine point sharpie, sharp pencil or even an exact-o knife, you just need to be able to rely on it to make sure you get the ground glass in the right spot.
The older wooden holder that I used allowed form the metal sleeves that hold the film in place to be pulled out and then I was able to seperate out the thin wooden divider septum without breaking the holder. I dug through my 'random parts and glass' bins until a found a decent sized ground glass that can be used (or cut down).
I ended up epoxying two really thin wooden strips to the top and bottom of the holder which allowed the glass to lay on it and then used a super thin bead of adheasive caulk around the glass to hold it securely in place. Use your dowel rod before you caulk or epoxy in place to make sure you are going to secure the ground glass in the right plane to ensure decent focus.
Certainly shoot a critical focus test before relying on it for anything important.
Let me know if you have any other questions, hope this helps!
Dirk
