Yes, you're right. You don't want to attach the two fronts to each other. You don't want the new sts at the tip end of the needle either. You want to begin by knitting through the right front flap sts (That's the right as if you were wearing it.) Then you want to cast on sts and finally knit through the sts on the left front. Now you'll turn and work back on the WS or inside row. The turn at the end of the row is so that the front flaps don't join.
Those right front sts should be at the back end of the needle, then the cast on sts, then the left front sts at the tip of the needle. This video
is for a crocheted collar or hood but the stitches begin on the same side of the cardi as your held sts. She crochets all the way around whereas your directions are to cast on over the back of the sweater.