Step 4Assemble and square up frame
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Assemble the 2x4 frame, using the 3" screws to attach the shorter 2x4s to the longer 2x4s, "capping" the ends of the longer 2x4s (refer to diagram). Don't worry too much about keeping the frame square, just suck the screws up tight. Once you have the frame assembled, measure from corner to corner diagonally across the frame. Push and pull the frame, checking the measurements frequently, until the measurements are the same (about 88-3/16" in this case). When they are, the frame is square. Yes, I know, it could be a trapezoid, but if your components are reasonable close to the right length, it will be square enough. This is one of the processes used to square up anything larger than your largest reliable squaring device, from foundations to windows, and no math is required. Use one of the 4'-7" 1x4 slats attached with two 2" screws across one of the corners of the frame to keep it square temporarily.
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