Sometimes you almost need a third hand to assemble a project. Especially working with large pieces of plywood. To help me keep the pieces aligned, I use assembly blocks made from scrap. I made these blocks from medium density fibreboard (MDF) with intersecting dadoes cut in the middle (pieces of plywood or solid wood would work as well). Just set the work pieces in the assembly blocks or place the blocks on top to hold the pieces in place until the clamps are installed.
Ted's Woodworking - Guide eBook