Yarnover 3 times. Insert hook into the base chain of the foundation single crochet. Yarnover, draw up a loop. Make 4 more dc into that new base chain to complete the shell stitch. The shell stitch is complex enough to bring up another technical idea.
Rows can begin or end either at the single crochet or with the middle dc of the shell.
You need to think about where in the pattern stitch you are starting, and go from there. This is done in a special way. Do not turn at end of second row. Thread over, insert hook in same place as last double crochet was made and draw up a loop. There are three loops on hook.