There are a few reasons you might choose to use a provisional cast on in your knitting. Perhaps you're not sure if you want the final edge of your piece to be knit or crocheted, or maybe you need to try it on before committing to the length. Whatever the reason, a provisional cast on is a great way to create a removable placeholder for live stitches.
To do a provisional cast on, you'll first need to create a slip knot and place it on your needle. Then, working with either waste yarn or another spare needle, pick up stitches along the edge of your work. Once you have the desired number of stitches picked up, cut the yarn leaving a long tail. Now you can remove the placeholder and begin knitting (or crocheting) as usual with your live stitches!
When you're ready to finish off your project, just undo the provisional cast on and graft together the two pieces using Kitchener stitch.