Cut legs and crosspieces to length; sand with 150-grit sandpaper. Arrange one leg assembly flat on the workbench and mark the position of the crosspieces. Drill and countersink screw holes, apply glue to ends of crosspieces and attach legs with screws. Repeat for the other two leg assemblies.
Measure and mark leg position on sink bottom; attach legs with screws driven through from the top. Position shelf onto lower crosspieces. Attach with counter-sunk screws.
Drain and liner
Measure and mark position of drain hole on sink bottom using a compass. Drill 1/2”–diameter pilot hole, then complete cut with a hand-held jigsaw fitted with a fine-tooth blade.
To cut and fold liner, you'll need tin snips and a makeshift bending brake, such as a couple of lengths of wood. Or get a sheet-metal fabricator to do the job. Mark the cut and fold lines onto the galvanized sheet with a ruler and permanent marker. Cut metal, then cut the corner as shown. Sandwich metal between wood bending brake pieces so edges of wood line up with fold line on metal. Secure with C-clamps, then bend metal to almost a 90 degree angle. Repeat on remaining three sides, then drop liner into sink. Mark cut-out for drain from below, remove liner and cut hole to size.
Position liner in sink, paying attention to corners. Attach with screws driven into top edges of front and back pieces. Install drain assembly and apply a thin bead of silicone sealant to each corner.
Seal all wood surfaces with three coats of oil-based polyurethane. Stain wood first if desired.