After completing your twisted rib hem, the pattern instructs you to cast off the body stitches "in pattern". This is how it's done.

Welcome to Lesson Seven
Casting off the body and onwards to your sleeves. Let's do this!
Casting Off in Pattern
Reconnecting the Sleeve Stitches
In this lesson we will get your held sleeve stitches back up on your needles and then pick up stitches to bridge the gap at the underarm. Watch carefully to see how to continue the ribbed detailing down the underarm and to catch our tip for avoiding holes at the join points.
Completing the Sleeves
This lesson covers: The Magic Loop technique, resolving the extra underarm stitches, a quick demonstration of K2tog and SSK decreases, working the rib detailing, managing yarn ends for striped sleeves, and a tip on the needle size to use when casting off your sleeves. There's a lot to do, so let's dive in!