join yarn for sleeves

I’m ready to start the sleeves of my cardigan! I’m perplexed by the instructions:

Slip 45 sts from waste yarn to larger needle. Join yarn at center of stitches cast-on for underarm, pick up and knit 5 sts, work in ribbing as established across 45 sts previously on waste yarn, then pick up and knit 4 sts in remaining cast-on stitches.

So I slip the stitches from the waste yarn back onto my needle. Then do I pick up and knit 5 stitches ? Or do I start in the middle of the row ("join yarn at center of stitches)? If I start in the middle, do I knit until I get to the middle, then pick up and knit 5 new stitches? What does it mean here to join yarn at the center of the cast-on stitches?

To take it in order you would start with a new end of yarn in the center of the cast on sts at the underarm. Leave about a 6" tail to weave in later and pick up 5sts. Then knit across the 45 sts that have been held on waste yarn. Since it’s difficult to knit directly off waste yarn, your pattern has you place those sts onto a needle. You’ll knit around all the held sts until you come to a small gap at the underarm. This is where you’ll pick up 4 more sts at the underarm to complete the circle.

Here’s a video for picking up sts in case you need it. It also shows picking up an extra stitch to close up the gap between the picked up sts and the held sts. That extra stitch can be knit together with what were held stitches on the next round.