I’ve made an error in my moss stitch and want to repair. Once I drop the stitches down how do I ladder back up?
You pick up from the front for the knit stitches and the back for purls. I use a crochet hook, but you can do it like Amy does in this video if you prefer.
Jan is right but a little more detail might help you.
Determine if the first stitch you are picking up is to be knit or purled. There are two ways to do it, with one you always pick up the stitches as though they were knits. If as you are looking at it, the stitch needs to be purled turn the work over, now it will look like it needs to be knit. Put the strand of yarn behind the loop, stick the crochet hook into the middle of the loop front to back and pull the strand of yarn toward you through the loop. Then take the crochet hook out of the loop, turn the work over and do it again.
You can also do it all from one side and then if you want to knit it you do as above but if you want to purl the stitch have the strand of yarn in front of the loop stick your crochet hook into the loop from behind and grab the strand and pull away from yourself through the loop. Take the crochet hook out and do the next stitch as a knit.