Casting On while knitting a sweater

I am a new stitcher trying to knit a top down sweater. I have put my sleeve stitches on a piece of yarn and now have to cast on an additional five stitches. I’m stuck on how to do it correctly…I know there is a simple solution but can’t find it anywhere. Help!

Use the knit or cable caston. Put the right needle into your left hand, cast on the stitches, then swap it back to the right hand and knit the next set of body stitches.