Cut a strip that's as wide as one of those triangles is high (from the center point to the flat side). If the stripes run selvage to selvage, then you want to cut parallel to the selvage. If they run horizontally across the fabric, then cut perpendicular to the selvage. Then cut the strip, starting at one corner, at a 45 degree angle down. Starting from the end of your previous cut, cut at a 45 degree angle up. Then down, up, etc. Keep going until you've gone down the whole strip. That's if you need many.
If you only need one block's worth, then cut two squares the same size as your central square there plus twice your seam allowance, lining up the fabric so that it goes along the diagonal of the square. That's usually easy, since the lines will either go vertically, horizontally, or (if you're lucky) diagonally (45 degrees) with respect to the selvage. Cut the squares in half along the diagonal, and arrange them around the outside of your central square to make the block.