Removing a Provisional Cast-On
In order to remove a provisional cast on, you can simply un-do the last chain stitch and pull on the tail, unraveling the provisional cast on until your live stitches are exposed. I find that a little terrifying though, so I prefer to place each on my needle and then pull out the provisional cast on one stitch at a time. It may be excessive, but it gives me peace of mind, so below I show you how to do it my way.
Find the end of your provisional cast on with the chained stitches. This is the end to begin unraveling.
Un-do the last stitch so that you have an open loop.
Gently tug on the loop and unravel the chain. If your waste yarn is drawn through the first stitch, just pull it out and place that stitch on your needle.
Insert your needle from back to front into the next held stitch and then tug on your waste yarn to pull out that crochet stitch.
Here's what it looks like to have a few live stitches on the needle.
To help open up the stitch that I want to insert into next, I hold the next open loop firmly between my left thumb and middle finger, then pull it away from the work. That makes the stitch below much more visible so it's easy to insert my needle.