Hi, if you have the pattern link, that would be helpful.
This might be obvious to you, but the first tip to setting in sleeves is to steam all the edges of all the pieces first. I also place a thread of yarn at the center top of the sleeve and match it to the shoulder seam of the sweater, then I can gauge stitches as I sew the sleeve in.
Here's a video from Berroco
If you can send the pattern, might be able to help further.