Gathering is a fun technique that you can use when you are sewing. It is created by using a long stitch on the machine and then pulling the thread, thereby pulling the fabric together. This works well on thin fabrics, but on thicker fabrics, you run the risk of the thread breaking. To prevent this, use a thicker thread like crochet thread and you will be safe from potential breakage.
It is a quick and easy thing to do to save you time of restarting your gathering process if the thread breaks.