I shall go ahead as if Stuart wants to cut square pieces of ply from bigger pieces of ply, so here goes...This isn't meant to be talking down to anyone, I needed this level of instruction when I made a piped edge velvet cushion for a stool..and two Elastplast..
Stuart, armed with an expanding steel tape, 10ft will do fine, go to a B&Q, Homebase what ever that sells 6mm plywood in pieces smaller than 8 x 4ft.
1. Look for a stack of 4 x 2ft, drag them all out, and start measuring across the diagonals until you find a piece with identical diagonals. ( A willing long suffering? partner will make this easier.)
2. This will be a square piece of ply. Keep hold of it and go to a rack of hardwood profiles/strip wood, and look for square edge rectangular hardwood 25mm x 6mm (ish), straight and unwarped, which is why I suggest hardwood. You'll need about 1.50m. Buy some good PVA glue, I favour Evostick Resin W.
3. When you get home, mark and cut the ply across one diagonal as straight and neatly as you can with a hand saw to form two triangles. (Put a layer of masking tape on the back face to minimise the tear-out.)
4. Now you are going to stick a length of the strip wood along the short edge of both sides of one of the triangles, in effect you are making a giant square so...
5. Pre-glue one side of each piece of strip wood cut to a length a bit longer than the short edge of the triangle and pre-glue one face. Pre-glue a 25mm strip both sides of the short side. This simply means laying on a thin film of PVA leaving a couple of minutes and then wiping off with a lint free cloth and allow to dry. This will allow final construction with less liquid glue, making things less likely to slide about.
6. After spreading a moderate film of glue on one side of each strip, stand the triangle upright on its short side ( A willing long suffering? partner etc... ) on a flat smooth surface and clamp the strips on each side pressing down firmly and evenly along their lengths against the same smooth surface.
A squirt/dribble of superglue at the junction between the pieces will let you relax sooner.
Clamp the ends that you can get at, then add one in the middle once they are holding all safely making sure above all, that the all slices of the wood/ply/wood sandwich are pressed firmly against the smooth flat surface.
7. When dry, unclamp, clean up any extruded glue and add three or four countersunk screws to reinforce the joint. You should have a 4ft long square to draw lines right across sheet materials at 90 degrees to the edge, or a guide against which to run a circular saw.
I do hope this will help.
Doug