Using a measuring tape, measure the length and width of the area where you want to pour your cement. Decide how thick you want to pour the cement. This is how wide your plywood pieces need to be. Purchase coated plywood from a lumber or home improvement store that is the correct lengths for your project.
Clear the area where you are planning to pour your cement and remove any grass and other plants in the vicinity. You will build your form here, since it will be too unstable to move after it is finished.
With a pencil, mark 1 foot in from either end of each long board. Turn one of these long boards up on its side edge. Butt the short edge of one of the shorter boards against the long board at one of the pencil marks. The short edge should follow the pencil mark vertically. Clamp the boards together and then hammer 3 to 4 nails through the back of the long board into the edge of the short board to secure them. Affix the second shorter board to the first long board in the same way. Line the second long board up so that its pencil marks are against the free end edges of the shorter boards and nail it into place.
To hold your form in place while you pour your cement, hammer a plywood stop pin halfway into the ground approximately every 2 feet, tightly against the outer sides of your form. Use a hacksaw to cut away the top of each pin so that it is even with your frame. Do not simply hammer them into the ground until they are level, as they will be too hard to remove later. You are now ready to pour your cement.