Make the diagonal square larger relax about the maths. You will find that pressing adding the applique etc all alter the middle diagonals. Even putting on the outside triangle corners you may stretch as you sew.
Cutting. Bis square down , donot forget to leave a 1/4 inch seam allowance above the points. When sew next outer edge on it may be easier as a newbie to sew with the pieced section on top. You can see where the seams cross just make sure you go just above ( forget seam allowance fo a few stitches past) .should be sharp point if you can follow my instructions.