How to Do Invisible Slip Stitch Seamless Slip Stitch
I have tried a few ways to do invisible slip stitch, this is the most simplest and efficient way I found.
It works for both regular single crochet (yarn over) and cross single crochet (yarn under).
You can even skip the turning chain of the new round to make the slip stitch even more invisible. Just make the first stitch taller by pulling the yarn higher to match the starting height of the new round.