Provisional knitting is a technique that allows you to knit in the round without having to start with a cast on. It's often used when you're not sure how many stitches you'll need, or if you want to change the color or pattern later on.
To do provisional knitting, you'll first need a crochet hook and some waste yarn in a contrasting color. You'll also need your regular knitting needles and yarn.
Start by crocheting a chain of stitches with the waste yarn and hook. The number of stitches should be equal to the number of stitches you want to knit in the round. Then, using your regular knitting needles and yarn, knit around the crochet chain until your work is as long as desired.
When you're ready to finish, carefully undo the crochet chain (being careful not to drop any stitches) and put all of the live stitches onto one needle. You can then proceed with whatever bind off or finishing techniques are required for your project.