Use any yarn of choice, and appropriate size hook. I’m using scraps of worstened weight yarn and a 5 mm hook. Finished size can be adjusted depending on how big you make it (which is why it’s a favorite pattern of mine) or how much yarn you have, etc.
In fourth chain from hook, make one double crochet. In the next two chains, make one more double crochet in each of the two chains. This means you have the turning chain where you skipped your first few chains, and 3 double crochets. This forms your first shell block.
Now, turn that first shell a bit, and slip stitch into the very first chain space (where you skipped the very first stitches). Chain 3, and make 3 double crochet into that chain space.
Do you see what we are doing? Each row adds one more “shell”.
Chain 6, double crochet into the 4th chain from the hook & into the next two chains. Turn work, slip into chain space that you skipped on the row before. Chain 3, and make 3 double crochets into the same chain space.
Slip into next chain space, chain 3, and make your 3 double crochets as before.
And just keep going, for as long as you want, changing colors whenever you like/run out of yarn.
The rest of this series will include how to turn the square into a rectangle, and how to do the decreases.