Welcome to Knitting Help!
You can do the provisional cast on if you want. Here's a video
for it that may help. The first method doesn't use the cable from a circular needle and may be clearer . When you've cast on the 12sts, the idea is to then slip the stitches off the needle onto the dpns and begin the Tree pattern.
I think you could also just cast on the 12sts as you usually prefer, join to knit in the round and when you're finished, use the initial tail of yarn to close up the hole at the center. It's just as easy as putting the sts from the invisible cast on onto a needle and pulling to close up.
Fabulous blanket that looks like it's going to be fun to knit!