The finish was done by first chipping off any old loose paint. You really need to do this, because a lot of these old windows were painted with lead based paint. Make sure you wear a mask and discard those paint chips properly.
I then used an oil based stain to darken the areas without paint.
The yellows, oranges and apricot colors came from playing with amber shellac. In some areas I used 3 coats, that I applied at random with a rag. In other areas I used only 1 or 2 coats.
I always cut my own mirror. For this project, I used reclaimed mirror from a 1950s vintage vanity. The vanity itself was long gone, but the mirror was still in good shape and it had some wonderful etching on it that I wanted to incorporate into these windows.
I use white silicone to hold the mirrors in place. My favorite way to apply it is with a caulking gun, and using at least twice as much caulking as what I need. I like the caulking to squeeze out around the glass a little bit to give it more of a rustic, handmade feel. The excess caulking sort of reminds me of the way they used to make old brick buildings, where the mortar would be allowed to squeeze out from the bricks.
I have done bunches of of mirrors like this, so if you have questions or comments be sure to post them. I would be glad to help you!