Measure the mirror on the wall to determine the length and width of frame (Image 1).
Using a table saw, cut the trim and molding pieces to size. Keep the table saw unplugged when not in use, especially when adjusting the blade or fence.
Now cut out a pocket or groove in the primary trim pieces to accommodate the edges of the mirror (Image 2).
Note: Figure your preferred finish for the frame first. It's easier to apply stain to the wood pieces prior to installation. Painting can be done after installation.
Add a sill board on the bottom edge of the frame (Image 1).
Cover any visible screw holes with a thin trim piece, such as 3/16" in depth, to make a false front and nail it into place using small nails.
Next, add the final molding trim pieces around the top and sides of the frame (Image 2).
Cover the nail holes with wood putty.
Add a bead of silicone around the mirror for added stability, wiping up any excess on the wall with a clean wet rag.
Paint or finish as desired.