Measure and Cut the Wood
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.