The process is actually pretty simple. First you’ll need to remove the wood panel that will serve as the window pane. This can be done several ways. Probably the best two ways that I can think of are to use either a circular saw or a router with a straight cutting bit. If you’re going to use a router you could make a simple jig to clamp to the door that would ensure you cut the exact size you want. The key here is to make sure you don’t cut too far into the corners. With both methods you’ll need to finish the corner cuts with a handsaw in order to get nice clean corners.
Once you cut out the panel you’ll want to sand the edges and treat the raw wood with either paint or urethane in order to protect the wood from rot. Next you’ll need a piece of glass thats slightly smaller than the window opening. I suggest making the glass approximately 1/4″ smaller in height and width to accommodate thermal movement in the wood. It’s important to note that most codes require tempered glass in doors. You may even want to purchase an insulated piece of glass (this could be quite expensive for a custom size).
In order to hold the piece of glass in place you’ll need to use some decorative moldings. The moldings will need to be sized based on the thickness of the door. For instance, if you have a 2″ thick door with a piece of glass that’s 1/8″ thick, you’ll need the moldings to be no greater than 7/8″ thick. You’ll also want to try and find a wood species thats similar to the native door species.
Nail the decorative trim into the opening on the outside of the door (you’re creating a sandwich effect, wood stop – glass – wood stop). You want to make sure that the molding is positioned so that when you insert the glass it’s centered in the door. Next apply a bead of silicon caulking around the inside of the molding. Set the glass inside the opening up against the first row of molding. Apply another bead of silicone caulking around the edge of the glass. Finally install the inside layer of molding up against the glass panel to complete the sandwich.
Once you’ve finished installing the glass you can apply a finish to the glazed wood doors to match it’s existing finish or you can re-finish the entire door. I hope this explanation offers sufficient information for you to try this interested door conversion.