Another method is to layer the two fabrics, RST, and sew all around the outside. Then you cut the block you want to be the outside square on the diagonal lines.
The easier way to do this is to cut the lines partially before you sew them together. Cut a small section of line one way with a ruler and your rotary cutter. Then cut the other way, but not through the center. Make two small cuts the second way. After it has been sewn, cut these lines to the corners with a small pointed scissors.
This method produces chopped off points, but all of the points are chopped off, and it looks great.