The shoulder cap would be the top of the sleeve. I’m assuming that those are bound off.
What I do for sewing in a seam is this: I tie short lenghts of yarn at the ends of the sleeves to the beginning of the armholes, just in a bow so it comes out easily, then I tie the center of the top of the sleeve to the shoulder seam and halfway down each side.
At this point I sew it together using a type of Mattress Stitch where I pickup the bar as in the video on the sweater side and the two side of the stitches on the sleeve side. Sometimes you might have to pick up more on one side than the other to ease it in, but it makes a nice seam.
After the sleeve tops are in, then use the mattress stitch to sew the sleeve and side seams. I usually start at the edges and work my way up so I’m sure the edges are even. If there’s any kind of mismatch, it ends up in the armpit.