Let's make some larger units. I find it very helpful to label each unit with post-it notes as you get them done, otherwise it may become confusing.

Sew one of the HSTs made with your background fabric and color #1 together with an HST made with your background fabric and color #2 together like this:

Make ten of these and label them "Unit #1"

Now make two sets of something we'll call Unit #1a that will look like this:

**What happened here is that though I was certain I'd made only Unit #1s when I made this mystery myself, one of my test sewers found something that showed I must not have. Hence the bizarre numbering as I attempt to fix my mistake.**

Now sew one 3" square of color #1 and one 3" square of color #2 together like this:

Make ten of these and label them "Unit #2"

Now, sew two HSTs of color #1 and color #2 together to look like this:

Make for of these and label them, you guessed it, "Unit #3"

Finally, sew two "Unit #3"s together to form a pinwheel like below and call it Unit #4