With frame removed, this mirror is 2300 x 760mm in size. The plywood backing should be size of mirror, plus 35mm all around to allow for strip that will form rebate for mirror, plus an extra 5mm to allow for expansion and contraction. Here, overall size is 2375 x 835mm. Cut with a power saw along straightedge.
The frame is made of 3 parts, with the decorative moulding built up with an outer frame of pine. Predrill, then glue and nail pine to moulding using 30mm nails.
Sand edge of packers and back edge of built-up frame to round them.
Cut composite frame to length with a 45° angle at each end to form corner mitres. The long point to long point length is same as length and width of plywood backing, here, 2375mm and 835mm.
To join frames, predrill screw holes in top and bottom frames, perpendicular to the mitre. Ensure the joint will pull together without slipping.
Lay out frame components face-up on plywood back, using it to make sure frame will be put together square. Lay a piece of tape on back and apply glue to the joint, then hold joint tight by taping up sides. Apply a second piece of tape around outside of corner and another piece over the top.
Drive screw through tape across joint to hold tight. Repeat for all corners. Allow glue to set.
Carefully flip frame over with tape still in place. Peel tape away from back surface. Predrill, glue and nail on packers using 20mm nails. (Nail near edges so nail points do not poke through face of mouldings.) The joint between frame and packers should be quite prominent from sanding the edges.
Sand frame and edge of plywood backing, especially the corners, then give both a coat of primer, sealer and undercoat, tinted grey. Brush out well.
When dry, sand surface lightly, then give frame and edge of plywood backing 2 coats of high gloss enamel. Allow overnight drying between coats and sand lightly between coats.
When paint is dry, lay plywood backing and mirror on bench and align. Place frame over the top, making sure mirror sits inside rebate of frame. Move edge of whole assembly slightly off bench so you can access underside, then screw backing to frame using short screws spaced at approximate 300mm intervals. Stand mirror against wall in desired place and give your reflection a thumbs up!