I do mine with the easy angle ruler. I started by drawing out the finished size, then drawing the corner lines and taking my ruler and figuring out what the inner square needs to be (this is important). At one point I had a chart with each size.
Then I figure out what WOF I need for the corners and cut them with the easy angle ruler method, Hard to explain in writing, but if you've been using it then you'll be familiar with how that works. I'll go 1.4" wider than needed so I can trim. I will finger crease the center square and then line up the point of the triangle with that crease so I'll have it centered for when I trim.
One word of caution, you are starting out with a square that will end up on point. So if you have lines that you want to end up horizontal on the quilt, you need to cut them bias and then you might want to do the covered corners method.