Sawhorses are a dandy device for supporting tables and work surfaces. They are sturdy, light, easy and inexpensive to build, and fold up compactly for transporting in your vehicle.
First, determine how tall you want your table top to be. The standard height for a dining table (a table you will sit at) is 30 inches. The standard height for a work surface (a table you will stand at) is 36 inches.
Most folks will notice that the sawhorse pattern I use has no angle braces. I do this on purpose: level ground is rarely found at encampments, and without angle bracing the sawhorses "give" just a bit to conform to uneven ground.
I use milled 4 x 1 inch lumber for my sawhorses to keep them light enough for transport. Be sure to sand off or paint over any lumber grade markings for an authentic look. Use slotted screws instead of Phillips screws--Phillips screws were not around during the Civil War. Use brass hinges or paint your steel hinges flat black for a period correct look.
My trestle table was made from a 32 inch wide door, designed to be 30 inches high when set up. I cut the sawhorses 32 inches tall and 32 inches wide. Remember that with the legs of the sawhorses spread out, they will sit lower than the constructed height.

For one pair of sawhorses you will need:
Four sturdy brass hinges (and screws for hinges)
32 Slotted 1 1/4 inch brass wood screws (get a couple extra in case you drop some)
6 - 8 foot long 1 x 4 boards
Approx. 10 feet of 1/4 inch Manilla, Sisal or Hemp rope
Cut 16 32 inch long sections from the 1 x 4 boards (each sawhorse needs 8-32 inch boards).
Assemble the 4 sawhorse halves as shown in the drawing at right. Be sure to attach the screws at an angle as shown. This will give stability to the sawhorses, but still allow them to give slightly on uneven ground.
Lay a pair of completed sawhorse halves on the ground with the tops butted up against each other. Install a pair of hinges to the top boards to attach the two halves together.
Stand up your completed sawhorse and spread the halves apart until they are the desired height. Tie a section of rope around the bottom boards.