Hi and welcome!
You start with 45sts for your size and end with 104sts. I would do the ribbing and then increase 5sts, ~evenly spaced over the next row. You'll have about 79 rows for the increases so you could increase one stitch on each side every other row 20 times and then one stitch on each side every 4 rows 7 times. That'll give you increases of 5sts+40sts+14sts=59sts. When you finish the increases, knit even (without increases) until you get the length that you want or 21.25" according to the pattern (79rows).
I've never seen a pattern marked "Easy" that had no directions at all for shaping the sleeves. You might drop Royal Yarns a line and let them know.