The serger is a secure way to attach elastic to pants, skirts, shorts etc. First let me say that inserting elastic into a casing is easy but the elastic tends to roll or fold and can be very uncomfortable.
There are several different types of elastic gathering attachments or accessory feet available for most sergers. They do make it a lot easier. But if you don't have one yet you can do the following:
1. Quarter the garment - mark with straight pins.
2. Measure the elastic on the person the garment is for to be sure it will be comfortable and fit properly. If they are not available and depending on the stretch of the elastic make it approximately 3" - 6" less than the size of their waist.
3. Serge the short ends of the elastic together to form an circle.
4. Quarter the elastic and mark with straight pins.
5. Match the pins of the garment to the pins of the elastic.
6. DO NOT SERGE OVER PINS!
7. Starting at the back of the garment serge the elastic to the garment. NOTE - you will have to stretch the elastic so that they both will lay flat when approaching the needles.
8. When you get to the "back" again serge over the previous stitches and then serge off.
9. At this point you can turn under the serged edge and zig zag it down.
10. OR if you have a 5 thread serger you can stitch it down with a cover stitch. But again you will have to stretch the elastic so that it lays flat.
Since serger stitches have stretch - it will be just super dooper when done.