This first step of the mystery quilt will get your blocks for it prepared.
1. Using your background fabric cut the following:
3 squares that are 12.5" in size
3 squares that are 15.5" in size
2. Using a single fabric add a line that goes across at least 4 of the 6 squares in any direction.
Note: If you decide to piece the fabric int0 a diagonal direction on the 15.5" squares, you may want to cut the background fabric a little larger so that you have room to square the block up afterwards.
3. Using either of the neutral fabrics, add a border to at least 2 of the sides of the 12.5" blocks in order to bring the size to 15.5".
Some possibilities for adding borders include those I've shown below. How many others can you find to bring your blocks up to 15.5" unfinished? Remember you can do anything you want with the borders for the blocks, all the same or all different it is your choice.
4. Square all blocks to 15.5" unfinished.
Set aside till the next clue.