Cut five 2-by-4s to 3 feet 9 inches long. Lay out two full-length (8 feet) 2-by-4s on the ground on their 2-inch edges, then place the cut pieces of 2-by-4 between these two pieces, also on their 2-inch edges, with one at each end, one in the center, and one 2 feet in from each end. This should give you a 4-by-8-foot rectangle (the actual thickness of the 2-inch side is really only 1½ inches) with three supports spaced at about 2-foot intervals.
Drill pilot holes slightly smaller than the non-threaded portions of your 3-inch screws through the side of one of the 8-foot pieces and about ½ inch into the end of the cut piece, then insert a screw to fasten them together. Pilot holes keep the wood from splitting when you insert the screws. Drill two holes and insert two screws to attach each cut piece to both of the 8-foot boards. This is your platform's frame.
Align your plywood on top of the frame and screw it to the frame using a 1¼-inch screw every 12 to 15 inches. Fasten the corners first, then fill in the rest. Make sure the screw heads sink in all the way until they are flush with the wood.
Determine how tall you want the platform to be, then subtract the actual thickness of your plywood, which may be slightly under ¾ inches, from that height. Cut 10 pieces of plywood at that length for your platform legs. Take two legs and align the 2-inch side of one along the edge of the 4-inch side of the other so it forms an L shape when you look at it from the short ends. Screw them together in this shape, then do the same for three more pairs.
Turn your platform upside down and place your L-shaped legs inside each corner of the platform and clamp them in place with C-clamps. Drill holes the width of your bolts all the way through both the platform edge and the leg, with one bolt on each face of the L, resulting in two bolts per leg. Clamp your remaining two legs near the middle of the platform's edge and drill two bolt holes in each.
Drill holes with a 1-inch spade bit about ¼ inch into the platform's frame using each of the bolt holes as a center. These are called countersink holes and will make it so your bold heads are flush with the surface of the wood and not sticking out, which wouldn't allow you to attach multiple platforms together. Make sure the holes are deep enough for the bolt head and a washer.
Bolt the legs to your platform using a washer on either side of the wood to keep the bolt head and nut from sinking too deeply into the wood. Turn the platform upright again and paint, if desired.
Fasten multiple platforms together by crawling underneath and bolting them together with two bolts per platform side in the same way you bolted on the legs. Countersink holes are not necessary. Bolt together as many platforms as you need to create an adequate stage size.
Remove all the bolts to disassemble the stage and store the platforms and legs separately.