just get out and do it. are you using 4mm or 3mm ply.
lay 4 sheets of ply, with best faces to each other. mark a line 24mm from edge of each sheet of ply, or 32mm if using 4mm ply. then line each piece up with line, and clamp tight to a bench, use a bit of scrap ply on top and bottom so as not to damage the edge of the good sheets. i also had to put a couple of nails in the midle of the sheet to hold it down tight. then use a hand plane to get most of the material down then use a long sanding block, to finish off with. get out in the shed and start building befor the sailing season is over.