Anyway, if you're new to this, here's how...
Grab a scrap of yarn. A different colour is ideal and also a similar weight. I'm using super chunky here, so anything too skinny will just fall out.
When you get to the stitch you want to mark, before you make the stitch, lay the scrap of yarn alongside the previous stitch. Hold it in place with your thumb.
|View from the front side of your crochet|
|View from the top of your crochet|
You actually only need to hold it in place until you can get the hook into the stitch below, then it'll stay put. Yarn over and pull through and it will look like this...
Yarn over and pull through again to finish the stitch - I'm assuming you're working in DC (SC if you follow US terms), but obviously, this works whatever your stitch.
And that's it. Your stitch is marked and you can carry on with the round.
When you complete the round and get back to your marker, whip out the scrap of wool, wrap it round the work as you did above and carry on. The stitch that goes in where you've just removed the marker is the first stitch of your new round.