Picture frames have been around for centuries and there are 1000's of ways to build one. Normally I would not build a picture frame out of plywood. I prefer to use a nice hardwood or even an exotic wood but let's see if a picture frame can be built with a medium grade plywood.
The goal of this frame design is to end up with something that looks like it was not made of plywood and even something that may have hung in the office of Jules Verne in the 1880's. (Well, that's the goal)
Although plywood was invented in the 1860's, I'm not aware of plywood picture frames from that era. So this frame needs to have a non plywood look.
A medium grade plywood has one major drawback for this project and that would be the edges. Many designs of art, furniture, buildings, etc., highlight the natural edging of high grade plywood and it can look quite nice. But this project will use a less expensive grade of plywood to keep the cost down.
This design requires only minimal skills in tool use so just about anyone can build one.

Supplies

(1) 3/8 inch thick 4 by 8 sheet of sanded (or better) plywood
20" by 60" clear lexan or glass
Table saw or circular saw
Miter saw
Spring and wood clamps
Various sizes of button head cap screws
Matching threading taps
Putty knife
Waterproof yellow wood glue
Stainable wood filler
Sanding sealer
Various grits of sandpaper
Dark color of deck toner (or your favorite
stain)

Step 1: Determine Frame Size

Determine the size of your frame. This frame will accept a 20 by 60 inch print so will be rather large. However, any size you choose will be the perfect size!
First cut a 20 inch by 60 inch piece. This will be used to attach to the back of the frame to secure the glass and print. This back piece is finished and can be set aside for now.

Step 2: Cutting the Plywood Strips

Cut 3 inch wide strips to form the frame front. 2 top pieces and 2 side pieces. Cut all other strips 2-3/4 inches wide. When all pieces are glued with overlapping corners, you will end up with a pocket that will accept a 20 by 60 print and matching glass.

Step 3: Gluing the First Joint

Start by edge gluing one corner with a square to ensure a true 90 degrees. A flat table is a must for accurate assembly. I like using the granite top of our kitchen island. Use just enough glue for minimal squeeze out. A small piece of wax paper protects the table. Keep the joint tight while placing the spring clamps. Wait at least one hour for the glue to dry and then repeat for the other 3 corners. This butt joint will be just strong enough until the 2nd layer is glued in place.

Step 4: Overlapping Strips

The 2nd and 3rd layers overlap each corner and add considerable strength and rigidity to the entire frame. Three main layers are sufficient but add more if desired. Clean off any glue squeeze out and allow 1 to 2 hours between clamping.

Step 5: Main Frame Body

The basic frame structure is now finished and is ready for 2 raised top pieces. The first top layer pieces will be cut with 45 degree angles and glued in place. A total of 4 top strips are added and the basic design is finished.

Step 6: Finishing the Edges

Now use a sharp putty knife and apply wood filler to all edges. This step actually goes pretty fast once you get the hang of it. Use just enough to smooth out the edges and to minimize sanding.
Now is a good time to drill and tap the holes while the wood filler dries. After the filler is dry, sand the entire frame and apply sanding sealer. Then apply the first coat of stain.

Step 7: The Art of Staining

Staining wood is always a little tricky to achieve great results. If the wood is porous (as in this case) a sanding sealer really helps to even out the stain absorption. Gel stain can sometimes be used without a sealer if you care to try that.
This frame will probably need 6 to 8 coats of stain to achieve an aged look. When applying each coat, allow the stain to slightly pool in corners and edges. This will help achieve the aged, dirty appearance we are looking for. The photo here is after 5 coats.

Step 8: Embellishing the Front

Once the finish looks good and is thoroughly dry, apply a coat of wax and buff lightly. Then screw in all of the button cap screws and frame front is now finished.

Step 9: A Finished Frame

After attaching all screw caps, cut a 20 by 60 piece of lexan or glass. I prefer lexan as it's unbreakable. Lay down your lexan and print. Then attach the 20 by 60 inch plywood back that was cut in the beginning. Any heavy duty frame hangers will work fine. Just use what you like.