This is actually not as difficult as one would imagine. Let's say you are attaching a sleeve. You would align the two pieces together first with pins. But remember to remove the pins BEFORE they approach the blade. It is a costly repair if you chip the blade or damage one of your loopers.
Just like sewing in a circle - you knotch to start. That means cut a section about 1/8" - 1/4" in by about 1" long. Lift the presser foot, advance the fabric so that the end of the cut is at the neck of the blade. Serge all the way around and then finally serge on top of some of the previous stitches and then serge off. If you are serging in a circle - you serge a little on top of previous stitches, then lift the presser foot and move the fabric away from the needles and serge off. Secure the overlap with seam sealant and then trim excess thread carefully.
Most sergers are not free arm - so that means you approach serging in the round a little differently. It is sort of like if you placed a can on it's side with the open side facing the machine and the excess above the machine. Not a good mental picture but I hope you understand. Please don't try to sew a can!
NOTE - WHEN STOPPING THE SERGER TO ADVANCE THE FABRIC - MAKE SURE THE NEEDLES ARE IN THE FABRIC!!! IF NOT YOU MOST LIKELY WILL GET SKIPPED OR LOOSE STITCHES.
In the future I will place a graphic and more detailed instructions on serging in a circle. As well as serging inside and outside corners. Coming soon!