Someone who shall remain nameless but is very dear to me asked me about provisional cast-ons. So here goes--in words without pictures.*
Provisional cast on is a cast on that you want to later take out. It can help if it's important to you that your cast-on and cast-off look identical (personally, I'm over that). It can keep you from having a seam and lets you knit from the center out if you like, as in this pattern.
Basically, you start with the same weight of yarn as your project, and a crochet hook in that is a size or two larger than your knitting needle. Pick a contrasting color yarn to make it easy to see later. Neon orange is perfect, unless you're planning on knitting a hunting vest; in that case, I recommend another equally obnoxious color.
First, make a crocheted chain of the number of stitches you'll be needing to cast on, then add about ten more to spare, so you don't have to worry about running out.
Then you start knitting into the back of the crocheted chain, one stitch at a time. Try not to twist your crocheted chain. The goal at the end is to be able to zip it off like the string at the top of pet food bags. Actually, I never can get those damn things off, but that's the goal anyway.
You're picking up and knitting on the chain just like you do when you add a ribbed band around a cardigan. When you're done casting on the necessary number of stitches, just keep going, following your pattern.
Later, you'll unzip the contrasting yarn and have what are called "live stitches." Be sure to put them on a needle at some point. As in right away.
That's it. Clear as mud?
*Writing about knitting is like trying to describe how to tie a pair of shoes. The Elann pattern actually seems to explain it fairly well.