Get 1 piece of fabric, start serging, adjust the differential feed down, note what number you adjusted to, move DF again, keep trying until you get the look you want. You will have to adjust the DF on most fabrics, I always test before using it on my project. If you are trying to make ruffles you will need 2 pieces of fabric. The bottom fabric will be the one that gathers. You can use both needles, and 4 threads. I made curtains this way. If you are wanting to gather 1 piece of fabric adjust downward. I hope this helps. If you want to make a ruffle on a collar, hem, etc. move the DF upward and stretch the fabric somewhat, 3 thread or 4 is OK.