When making garments especially blouses/shirts attaching sleeves can sometimes be a challenge. After you have already attached the top or cap of the sleeve then you most likely will serge down the side closing the sleeve and the side of the blouse/shirt. Line up the previously serged seam and secure with washable gluestick. (If you use a pin make sure it is away from where you will be serging - it may damage the knife or the looper if you serge over it....)
USE YOUR LEFT HAND TO HOLD AND GUIDE THE FABRIC SO STITCHES ARE FORMED AT THE INTERSECTION. As the fabric is advancing towards the needles and knife you will notice that it is moving away from the needles/knife. Watch closely and use your left hand to make sure the fabric is being serged. I repeat----- your left hand is important and useful. The same technique applies when serging an inside corner, outside curve or circle.