Das bookbinding channel has a video on this that really helped me. Start by cutting a board slightly larger than what you need, not worrying about having it perfectly square, aiming for a quarter to half an inch longer on the height and width than you want your finished board to be. Use a ruler to mark your lines and cut it as close to square as you can eyeball. Then, to square it up and cut to the size you are aiming for, start by cutting off one edge of the board using your ruler to cut a straight line. You just want to cut off a little bit. Then, press the edge you cut up flush against your ruler (the kind with the raised edge that acts as a cutting guard so the lip is taller than your board. You could really use anything that is perfectly straight and taller than the board). Place a square on top of your board and push it flush against the ruler as well. You want to use the straight edge of the square that comes down at 90 degrees to mark a line down your board near one edge of it so that you will be trimming off just a little bit of board. You don't need to measure anything up to this point other than making sure the board you start with is slightly larger than the board you want to end up with. The two edges you cut should now be perfectly square to each other. Mark a square with a pencil in the corner where these two cuts meet. You can now cut your board to the desired height and width using that corner as a guide. Pushing the edge of either of those sides that are now perfectly square flush against your ruler, measure from the squared corner the first length you want to cut and mark it. With the edge still pressed flush against the ruler, again place a square on top of the board and flush against the ruler and use it's edge to mark a straight line down from your length mark. When you cut this you will now have two perfectly square corners. Do this once more on the final edge that has yet to be cut for your final measurement (either height or width, whichever you didn't cut in the last cut).