E.G., a 30″ x 40″ painting:
1. Get your supplies at lowes or homedepot or whatever
-2 x these for the outer frame:
https://www.lowes.com/pd/1-1-2-in-x-8-ft-Pine-Wood-Lattice-Moulding-Actual-1-5-in-x-8-ft/1000443635 (or deeper than your painting**)
-2 x these for the inner frame: https://www.lowes.com/pd/EverTrue-1-1-8-in-x-8-ft-Pine-Wood-Lattice-Moulding-Actual-1-125-in-x-8-ft/3042181 (or less deep than your painting**)
-Clear (or whatever color you want) and Black wood stain like this:
https://www.lowes.com/pd/Cabot-Tintable-Solid-Exterior-Stain-Actual-Net-Contents-7-25-fl-oz/5001615019 (or use sharpie or acrylic paint or whatever)
-Wood Glue
-Clamps
-Miter saw (if you want to miter) or just a good hand saw
-Sand paper
**Make sure the painting is somewhere between the depth of the lattice strips:
They sell lattice in various depths, so the links I pasted above would be good for a 1 1/4″ deep painting.
2. Measure and cut
That diagram shows you the general idea of where to make your cuts and how to line up the strips, but in reality those exact measurements won’t be perfect. So actually line up the lattice strips and mark them with pencil, then cut along the line. Err on the side of too long; you can always shave more off. Gaps are bad.
3. Sand the cut edges and stain them. You can use sharpie marker if you don’t feel like waiting for the stain to dry…
4. When the stain is dry, run some wood glue along the bottom edge of the black lattice and clamp them to the stretcher. Those will be the inner frame.
When that’s dry enough (like the next day) do the same with the natural stained lattice. Use as many clamps as you can to hold it together. You can even run twine around to make it super tight, but it’s probably not necessary unless you only have a couple of clamps.
Once the glue is dry, it’s pretty solid. You can drill small holes and hammer brads in, then cover the openings with wood putty but that’s a lot of hassle.
If you’re feeling adventurous, you can try mitering the corners. Use the pencil measurement method, then cut with a miter saw. Make sure you have extra wood since you will mess up the first time. It’s really not necessary though.
Good luck!