Hmm… I’ve looked at quite a lo5 or video tutorials trying to find a clearer one but I haven’t found any better. Maybe someone else knows a clear tutorial.
Now would be a good time to put in a life line with some smooth spare yarn or embroidery thread. This is a great resource for knitters new and experienced alike and well worth the time. If your folded neck doesn’t turn out right you will be able to go back to this life line without losing all your stitches.
If you’ve never done a life line before you might want to try it out on a small swatch, like in the tutorial.
If you follow the first stitch all the way down its column from the needle edge to the cast I edge and place a marker or safety pin or piece of scrap yarn through it, when you fold the cast on edge to the inside you will easily identify which the first cast on stitch is you need to join.
I would just pick up the cast on edge and not worry too much about if you got it exactly the right way. Learning is about trying, not always getting it right and then improving as you go. I did some quite random pick ups and increases on my first knitted sweater and no one knows. So long as you don’t drop stitches your sweater won’t fall apart. This join is all on the inside so going to be hidden anyway.
You could try it out on a small swatch if you have another pair of needles and a spare ball of yarn - it doesn’t need to be in the round, doesn’t need to be the same size needles, just cast on say 10 or 20 stitches and rib as many rows as needed to be able to fold it over then have a go at it and knit of on a few rows beyond the join to see the effect.
I use lots of swatches when I come across a new technique, you can practise without worrying about ruining your project.
Or, as salmonmac mentioned, you can skip this join and just continue with your pattern. Then at the end you can sew down the folded neck edge to the inside. Perhaps have a look at some videos on how to sew a knitted hem and see if you would feel happier to do this?