Cut two pieces from each of the two 8 foot pine boards to 43 inches. Set aside the 10-inch mill end pieces for another project. You should have four 43 inch pieces. Lay these on the floor between the two remaining 8-foot pine boards with the wide side flat on the floor. Butt the short boards up to the long boards at the top, bottom and in the middle two feet from each end. You should have a rectangular box with two cross bars.
Load your stapler with 1/2-inch staples. While holding the butted boards tightly together at the corners, staple across the seams with four or five staples. It helps to stand on the boards once you have them butted firmly. Do this at all seams joining the boards together.
Gently flip the frame over and staple all the seams on the other side.
Cut your 1 foot-by-2 foot piece of plywood in half to create two 1 foot-by-1 foot squares. Draw a line from one corner of each square across the board to the opposite corner and cut along these lines to create triangles. Position the triangles over each of the outside corners of the frame a 1/2 inch from the edges and staples these down to the frame with a dozen staples each.
Remove the 1/2-inch staples from your stapler and replace with 1/4-inch staples.
Lay your fabric on the floor next to your frame. Roll it out or unfold it so it lays flat on the floor like a rug.
Set your frame on top of your fabric. The plywood corner braces should be facing up. Pull about an inch of fabric up and fold it over the frame along an 8 foot side so that the leading edge lays on the top (up side) of the frame. Continue stapling down one side.
Move to the opposite 8 foot side of the frame and stretch the fabric taut toward you away from the side you have stapled. Begin stapling that side at one end while pulling the fabric taut then up and over the side.
After three or so staples, move to the other end of the flat and pull across and down away from the staples you just put in. Continue up the side until you've secured it with staples.
Move to one of the 4 foot ends/sides. Stretch and staple the fabric in the same fashion.
Move to the last side and stretch taut and staple.
Mix one part Elmer's Glue with one part water. Mix well.
Flip the frame over and generously paint all the fabric with the sizing. Let air dry.
Paint on another coat of sizing and let it dry. Notice that the fabric becomes taut and rigid as the sizing dries.
You may now paint the flat as you wish using acrylic or latex paints. Do not use combustible oil-based paints. Let it dry.
Flip the flat over and put one of the 3-inch nails into each corner. These will be used to lash the flats together.