Popcorn stitches all use the same process to create, regardless of what kind of basic stitch you're working with. They can be created with any stitch except the single crochet, but the double crochet stitch is most commonly used.
To create a popcorn stitch: work a group of 3 or more stitches, we'll say double crochet stitches for this example, into one stitch or space. Remove the hook from the working loop you've been using and insert it into the first double crochet in the group. Pull the working loop (that you dropped before) through where you now have your hook. Your popcorn should pop to the front. Secure it into place with a chain stitch.
Tip: When working on the wrong side, you can insert your hook from the back to get your popcorn to pop to the correct side.
This technique creates a 3D effect and can be used to create numerous designs and patterns. In my photo, I made a diamond.