Since ambidexterity is unfortunately not one of my strong suits, the quick start video and tutorials on this site are from the perspective of a righty. For lefties, switch which hand holds what, and flip the direction that you crochet. Easier said than done, so for more guidance, here are some lefty video tutorials from other creators plus my extra commentary:
What is amigurumi, and what do you need to have and know to crochet amigurumi?
Same video as above, but she explains this specific piece at 2:22. She breezes by how to single crochet, so see below for that tutorial. Note that she uses a bobby pin as a stitch marker in the last stitch of the round instead of the way the kit is designed, which puts the stitch marker in the first stitch of the round.
This tutorial explains how to single crochet stitch, but on a flat piece. The difference between single crocheting in the round vs. a flat piece (which is crocheted in “rows”, not “rounds”) is that for a flat piece, you add a chain stitch at the end of the row and “turn” it to create height and properly position yourself to crochet the next row.
Since we’re crocheting in “continuous rounds”, you don’t need to chain stitch or turn at the end of the round. I just wanted to clarify the difference because this video does talk about adding a chain stitch at the end of the row, which you should ignore.
Don’t be startled by the fact that this tutorial shows you how to single crochet increase on a flat piece. Like all crochet stitches, it’s exactly the same way you’d single crochet increase on a round piece.