Developed initially to teach my group crochet provisional cast on, this turned into a rather interesting and functional dishcloth. I think you could also use it for an afghan square, if you adjusted the end row counts to get it to the size you want, but I can’t guarantee that will work for you.

The recommended cast on is 45 sts, but it would be easy to alter that, and change the edge counts.

There is an extensive photo-intense TUTORIAL for PROVISIONAL CROCHET CAST-ON on the end of the pattern.

I do not define the basic stitches.

They include
k, p
ssk--slip one st, slip another st knitways, return to left needle, and knit the two together through their back legs.
k2tog--knit two together
yo--yarn over
k1b--knit one in back leg of stitch
sl1, k2tog, psso: slip one stitch knitways, knit the next 2 together, then pass the slipped stitch over the two that are knit together.
sl2, k1, p2sso: slip two knitways, knit the next st, pass the two slipped stitches as a unit over the knit stitch. A double decrease.