It's Shilyn Joy !

I hate sleeves, haha :P

So, I don’t know if you know what a dart is, but basically, on the pattern it looks like a triangle (that you would sew together), and what it does is it adds some shape/take in whatever you’re darting. For example, I might add two darts under my bust to my waist, so it would be smaller around the waist, but still larger around the bust, without needing to cut more than one piece.

So when it says to stitch the dart seam on the sleeve, hopefully you kept your pattern…You can draw the triangle onto the fabric, or just measure it out (I suggest tracing/drawing from the pattern, cause if you’re like me, I measure crooked a lot since I don’t use patterns often!)…
Fold the sleeve piece, right sides in, directly in the middle (where the point is!) of the triangle/dart…and match the dart lines on each side…
Sew on the lines = you just darted the sleeves!

Then, you would just cut the excess off about 1/2” from where you just sewed the dart.

Hope that helped, and wasn’t confusing ! >.< :P