After the cast-on rag is hung, knit in waste yarn. This allows you to separate the live stitches (that will be kitchener stitched to the arm, or you can rehang the live stitches and knit it directly later) from the cast-on rag.

This pattern calls for +10 armpit cast-on stitches. I hung six total on each side of this front panel. (half of 10 for each side, +1 stitch for selvedge).

Use any loop you can grab from the cast-on rag and hang. If the pattern calls for 10 armpit cast-on stitches, hang half on each side of the panel +1 (the +1 will get consumed when you mattress stitch the panels together.)

I made a bunch of these little crochet bits to use as cast-on rags. Hang them for the armpit cast-on stitches, add a piece of waste yarn so it will separate from the knitting later, and hang a weight. It is ready to knit.