First jumper - next step

[Newbie77]

Hi all, I've recently taught myself to knit from a beginners book. It's taken me a while, but I've finally finished the parts to my first jumper. I have the back, front, and two sleeves. The pattern now says "Using mattress stitch seam, join right shoulder seam. With right side facing and beginning at left side of neck, knit up 17 sts&