Tie a slip knot in some waste yarn and place it on your crochet hook.
Tension your working yarn so you're ready to crochet a chain.
Place your knitting needle between your hook and your working yarn.
Crochet a chain stitch which will now be looped around your knitting needle.
Return the working yarn to the back of the knitting needle, so the needle is once again between your crochet hook and the working yarn.
You can then chain as many stitches as you need, returning the working yarn to be behind the knitting needle after each chain stitch.
Once you have as many stitches on your knitting needle as you need, chain a few extra stitches without having your needle in the way. On the final chain stitch bring the yarn all the way through the loop to create a knot. Adding these extra chain stitches helps to easily identify which end of the waste yarn you'll need to unravel from.
Using the yarn you'll be using for your project, knit into each of the stitches on your knitting needle.
When you're ready to unravel your provisional cast on and return the live stitches to your needle, simply undo the knot at the end of the crochet chain where you added the extra stitches. Gently pull on this yarn to unravel the chain stitches and access the live stitches. Carefully place these live stitches back on your knitting needle.