I’d join the yarn and sc in the same stitch as the join. Following with two sc in the next stitches seems off since the rest of the round is (sc 2x, inc). What are you making? I see it’s in a book but sometimes the patterns are also available singly online.
Join the yarn by pulling it through the top of a dc, ch1, sc into that same stitch. “At base of chain” means to work into the stitch that the chain comes out of. This probably only happens at the beginnings of each round.