Divide the finished block into two
A four-patch is a two-by-two grid of equal squares. Each square finishes at half the width of the finished block. Add seam allowance on both sides of that square to get its cut size. The finished block size is measured after the block has been sewn into the quilt.
Cut a six-inch four-patch
For a 6-inch finished block, each square finishes at 3 inches. With ¼-inch seams, cut four 3½-inch squares. Once assembled, the block should measure 6½ inches before you join it to other blocks. That extra ½ inch is for the seams around the outside of the block.
Plan the fabric split
The total count includes every square in all the blocks you request. A typical two-color four-patch uses two squares of each color per block, so ten blocks need 20 squares from each fabric. Divide the total to match your design, then use the square cutting calculator to check how much fabric each group needs.