Sew all your white squares on opposite corners of each charm square. Trim excess over 1/4" seam and press seams open.
Now you are ready to sew your block together. Layout your block as pictured below and sew. Start by sewing into sets of two and then into blocks of four. Press all seams open. Then sew your left and right blocks together and then the top and bottom.
You will have a finished block like below.
Repeat with all sets of two patterns of fabric. You should end up with 20 blocks.