Use a cutting mat, a rotary cutter, and a plexiglass ruler with a 60 degree angle marked. Cut a strip. Line up the 60 degree line with the fabric edge and cut. Align the edge of your ruler with the angled side and make a cut the same width of the strip.
Cut a strip of fabric to the desired width and with a rotary ruler's degree angle, align with right edge. Make sure the ruler is allowing enough fabric to be covered. Rotary cut along the edge of the fabric aligned with the ruler. Repeat the process when you have a desired size of diamond.
To cut a diamond quilt block, first ut a strip of fabric that's 1/2 inch wider than your diamond's finished height. Next, align a ruler to a 30-degree, 45-degree, or 60-degree line with the left lower edge of the fabric. Rotary cut along the right edge to create one of the diamond's angles.