I cheat. I add 3-5inch sashing all around the original square. I then place my 15 inch square on top but tilted to the angle I want. I cut around the edge and it is ready to go. If you don't have a large enough square, cut a paper or cardboard template to trace around with a pencil. You can then use a rotary ruler and cutter to cut on the lines. The only draw back of this method is the wonky block edges are not on the straight of grain so be careful not the stretch them when sewing them together.
There are "wonky" block settings on the net if you want to piece in the angles and maybe someone else can give you a link.