Use a utility knife to break the paint seal and begin separating the molding (Image 1).
Use a small, flat pry-bar to separate the wooden window stop molding from the door frame (Image 2). If possible, try to separate the molding from the frame without breaking the molding. (In our case, the molding was so deteriorated that it came off in pieces.) Remove the window stop molding and set it aside.
Remove any glazing point holding the plexiglass in place and remove any old glazing compound that would prevent the plexiglass from being removed.
Gently remove the plexiglass, taking care not to damage the door frame as the plastic is removed.
Clean off the door frame surface with a putty knife or scraping tool as needed. Take care not to gouge the wood surface.
Remove any remaining glazier points.
Clean off the door frame with a light cleaning solution and let it dry.
Double check the measurements of the glass and opening, then carefully insert the new glass into the door-frame opening (Image 1). Press the glass down to bed it into the glazing compound.
Press in glazing points around the perimeter of the frame to secure the glass (Image 2).
Apply a thin bead of glazing compound to the exposed side of the glass at the frame (Image 1).
Re-install the wooden glass stops, or add new wooden stops as needed. Use a brad nailer to secure the wooden stops (Image 2).
Paint the wooden stops. Painting the wooden stops down to the glass ensures a paint seal between the glass and the wood. It's a good idea to prime new wooded stops prior to installation, when possible.
Using a hammer can be dangerous around glass. The nail gun is a better choice.