There is nothing easier than making string blocks! Make the bleached muslin backing (please wash before sewing) at least 13 1/2" square and trim up blocks to 12 1/2" after pressing. Here is a quick tutorial to make them.
Here is the link to the challenge guidelines and deadline:
2018 Charity Quilt Block Challenge
12 1/2" unfinished quilt blocks by Quilters Cache
Split Star Block Pattern
Twin Star Block Pattern
Card Trick Block Pattern
Bento Box Block Pattern
Box Kite Block Pattern
Faithful Block Pattern