Getting things square?
3' on the backer, 4' on the top board then 5' on the angle from 3' point to the 4' point makes the first board square.
For a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides: a2(3) + b2(4) = c2(5)
From there you can measure for the rest then at the end measure from corner to corner (they will be equal) and adjust.. you'll have the squarest picnic table on the block.
You can try this simple trick. Nail a jig or form to a flat surface like a piece of 1/2-3/4 inch plywood laying on the floor. The pieces can be 2x2 inch stock or really whatever you have lying around that have the length that you need. Using a framing square on the corners to lay it out. It will have to be the size of the top of the table so that the top will fit inside. Four pieces to form a rectangle in the dimensions of the top. Use the square to get the angles on the corners right. Lay the pieces for the top, bottom up, in the jig with the correct spacing. Then you can lay the cross pieces in place (again using the correct measurements to position them and the framing square to get the angle right). Use whatever fasteners that you choose (galvanized screws work well) and a waterproof glue on the cross pieces. Let the glue set to give the top some ridgity and then fasten the legs as the plan suggests. Good luck and enjoy the picnic.....MJH
Also, I recommend stainless, rather than galvanized screws if you want it to last without any rust.
One more thing: a simple way to get a square angle is to use a sheet of plywood as your guide. They are square when you buy them. Lay the pieces onto the plywood on the floor/ground.
You know I always use electroplated fasteners, but read in a book just a couple of days ago zinc coated is better.. I, like you rp, haven't found that myself, I find the zinc get's knocked off easier especially with gun nails... hmmm? Goes to show you got to take everything with a grain of salt huh?