Knitting in round; adding extra stitch

I need a refresher! My knitting instructor once told me that, when knitting in the round, it’s wise to cast on one extra stitch to make the point when the first row is “hooked up” stronger. Can someone tell me what I do with that extra stitch? I’m dreaming about this extra stitch and it still isn’t coming to me! Thank you!

I just cast on the right amount of stitches. But I have seen instructions where the first and the last stitch of the cast on get knit together (k2tog) on the joining point. Depending on the cast on you might have some slack otherways (but with my cast ony (knitting on or long tail, mostly) I do not see the trouble, really so I never do that and never found a real problem before I knew about it.

What I do with it is slip it to the R needle, work the first round and knit the last st tog with the slipped st. Some people knit them tog at the beg of the first round - either way it works.

I’ve also read to cast on an extra stitch. Then knit the last cast on and the first cast on together.