Help with sleeves

I’m just about to join the sleeves and a bit confused. Is there a video or diagram to explain the process? I’m confused by the instructions. This is hopefully for my son’s birthday on Sunday - March 27 and I’m in a bit of a panic. Thanks for any help ! Connie

Welcome!
If these are set in sleeves, this is a good video which used the mattress stitch for a nice ~invisible seam:


If it’s a drop shoulder sleeve (straight across on the sleeve top instead of a sleeve cap) the same method applies but the seam is even easier to line up.
Can you give us a pattern name or a link to the pattern?