Cut a piece of 1/2-inch-thick plywood (smooth on one side) 2 inches larger than your screen frame all the way around. A solid baseboard is necessary for good printing.
Attach 1/8-inch tempered Masonite to the baseboard by gluing and screwing the edge to the top of the smooth side of the plywood. (You could laminate Formica to the plywood instead.)
Apply a couple of coats of polyurethane to seal the surface and aid in cleanup.
Attach two clamp hinges made especially for silk screen printing to the top of the baseboard.
Use hinges that have a wing nut that allows the screen to be inserted and tightened into the hinge. The hinge then allows the screen to be raised and lowered as necessary.
Decide how the art is to be positioned (vertically - the long way - or horizontally?) and how it is to be printed (are you going to pull the long way or the short way?).
Space the clamps so they hold the screen frame in the stablest way, about 3 inches in from the perpendicular side of the frame.
Screw the hinges into position.
Attach either angle irons or 2-by-2 blocks of wood to the baseboard on either side of the silk screen frame to ensure maximum stability and good registration for multicolor print runs.
Start by attaching the frame to the clamp hinges and placing the frame flush with the surface of the printing unit.
Butt the angle irons or wood blocks up against the sides of the frame so that the frame is snug.
Place them on either side of the frame about 2 inches down from the end that's not attached to the hinges.
Screw the angle irons, if you're using these, into the base of the printing unit.
Secure the blocks, if you're using these, with C-clamps.