The worktable portion is made up of four 2x6 boards with half lap notches cut into them. When the plywood is placed on the worktable, these sacrificial boards are cut through. The cuts are not very deep, so the boards should withstand many cuts. The boards are also inexpensive, so when they become unusable, just make more.
The first step to cutting notches in the sacrificial 2 x 6 boards is to mark out the notch. The notch should be as wide as the 2x6 board it will be joined with and as deep as half the width of the face of the 2x6 board.
Cut out the notch with a jigsaw. The legs of the sawhorse worktable are made of 2x4s. Cut the 2x4s to the length that's needed. The legs slide into sawhorse brackets (Image 1). The brackets have holes in the sides so you can screw the 2x4s to them (Image 2). The teeth on the top of the brackets clamp onto two parallel sacrificial boards.
The sawhorse worktable is a space-conscious unit. When you don’t need it, simply remove the two sacrificial boards not clamped into the legs, and push the sawhorse portions together.