Take a moment to look at the horizontal bars of yarn running above the dropped stitch, we call these "ladders." You want to make sure that you pick up each ladder in order, not missing one. This is critical to keeping things tidy and in pattern. Don't let all the ladders freak you out. Just focus on one at a time.
Use your hook to pick up the ladder stitch from behind: scoop up from behind, bring it forward through the loop. Continue picking up ladders in order, maybe repeating this out loud if it's helpful "scoop up from behind, bring it forward through the loop."
Sometimes the ladders at the top are quite stretched out. It can be easy to pick up out of order, so double check that you are picking up the next ladder in line. If you get them out of order, just drop down and try again. If you're not sure, you can check the back.