This is a must know stitch if you want to learn how to crochet. Go under the next stitch from your hook and pull up the yarn making a second loop on your hook. Finally, wrap yarn around the hook and pull through both loops on your hook.

Do one single crochet stitch. Then, find the small hole below the stitch you just made(Green Arrow). Single crochet one more time in that space.

Start by pulling up a loop under your next stitch. Pull up another loop under the next stitch(Green Arrows). Then, wrap yarn around your hook and pull through all three loops.