For a 12" block you will need:
(1) 12-1/2" square light
(4) 4-1/2" squares dark
Using a white, silver or yellow pencil draw a diagonal line from corner to corner on the wrong side of each dark square.
Place a dark square (right sides together) in one corner of the light square and sew on the drawn line. Trim the excess, approx. 1/4" from the seam. Fold back the triangle and press. Repeat with the remainder of the squares, and corners of the light square.
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You might use Snowball to showcase lovely, large prints; the corners would help to separate and feature the print. Or, the Snowball block is a wonderful alternate block, and can be effectively used with nine-patch grid blocks (those that are 3 x 3 arrangement of units), such as the Nine-Patch, Friendship Star, or Monkey Wrench.
You can change the size of the dark squares to suit your needs. For example, you could use it for a photo quilt, using the corners as a frame, or accent piece--in which case you might want the corners to be smaller.
Here's the Snowball used to actually act as a snowball, in a quilt I designed a few years ago. (Mittens are a favorite winter motif for me...)