Step 1: Wrap your yarn over your crochet hook.
Step 2: Insert your crochet hook into the stitch or space desired.
In the photo above, I'm working a straight row of double crochet stitches. I'll be inserting my crochet hook directly below the two loops that are beneath the stitch I am working. If you're crocheting rows, go ahead and do it that way.
If you're making a granny square, or another pattern that uses spaces, you'll insert your hook into the space directly below the stitch you are working.
When you're working patterns in the future, keep in mind that the pattern might call for you to work a double crochet stitch into a space or into a different stitch. There are many possible double crochet variations.