Build the table top out of one-inch by six-inch pine boards. Buy the straightest, least warped ones your lumber yard has to offer, but realize you'll be able to overcome slight warp-ages when it comes to gluing and clamping the boards together. Run the boards through a planer, if necessary, to attain even thickness and cut them to a uniform length. Lay them on a flat surface and apply wood glue to the joining edges. Use large wood clamps to hold the boards together while the glue dries. Six boards glued edge to edge will make a table top of conventional width.
Sand the tabletop smooth using a belt sander.
Cut eight two-inch by four-inch table legs to the desired table height and screw them into the sawhorse brackets using wood screws and a cordless screw driver. Trim the bottoms of the table legs using a compound miter saw so the legs stand square on the ground when the sawhorse brackets are extended.
Cut two-inch by four-inch boards slightly less long than the tabletop is wide and clamp them in the sawhorse brackets so the brackets with legs extended form four corner legs of the table. The support boards under either end of the tabletop provide lateral support.
Position the one-inch by six-inch board top on the four corner legs and supporting end boards. The end result is a quite stable though thoroughly portable picnic table that was not too costly to construct.
When the picnic is over, remove the trestle top and end table leg-separating boards, fold up the corner legs and store all for the next picnic use.