Sometimes it is really handy to use a piece of string as a marker in crochet. Working in a coil, especially, you may want to remember where the beginning/end of the round is, and make sure it stays roughly the same place each round.
If you lay the string between the last stitch of one round and the first stitch of the next, that marks the point. When you get to that point after the next round, bring one end of the string back after the last stitch and before working the first stitch of the next round.
As you continue, notice how straight the dotted line is that the marker makes. That lets you know that the rounds are beginning/ending in line. If you were off on the shaping instructions, the line of the marker would get jagged and not look right.
As the piece gets bigger, just pull the string along, letting go of the earlier markings (because we know they are correct already), and it continues to mark the more recent progress.
If the notion of marking BETWEEN stitches is not appealing, you can just as easily include the marker in a specific stitch by laying it on the stitch as you insert your hook to make that stitch.
Either way, when you are done, it is easy to take out the marker by just pulling one end.