I do not understand where he states place provisional cast on stitches onto a spare circular needle, create welt by knitting 2 stitches together (one live stitch together with one provisionally cast on stitch to the end of the round?
My…this is certainly unusual. 
Is this on the cast on edge? It sounds, for the most part, that’s done like a hem. Do you know how to do a provisional cast on? Once you have the provisional stitches on a circular needle you would knit with another circular so the provisional stitches remain on a circ. You could also move them to a circ if necessary.
The construction of this sweater is very fun and begins with a stockinette stitch hemmed cowl neck
from the Ravelry page for the sweater.
Yeah, it looks like Jan’s got it, you’re starting out hemming the cowl. I’ve used this method before but not on a neckline. Interesting idea and I may put it to use this way sometime. Using a circ is a good idea. As you’ll be knitting the stitches from this one it doesn’t matter what size it is as long as it isn’t larger than the needle you’ll be knitting with - in fact I like to use one at least 2 sizes smaller than the working needle, it just makes it easier for me. If this isn’t an interesting knit, I don’t know what would be. Have fun!
How to Knit a Seamless, Integrated Hem
Not a terribly great video but it shows what you’re doing. She knits it flat and uses two colors. It can be done in the round and if you’re using only one color, no problem. If you’re doing a welt it won’t lie flat like hers does and you won’t using a garter stitch row for turning. Otherwise, this is pretty much the same thing. Does the pattern specify which provisional cast on to use?