I just went through this last week! Finally decided to try out some of the features of my serger and couldn't find any info at first. Then I realized there is a separate booklet that has that info- it's a thinner, white booklet that is titled "Serger Techniques". According to that, it says to use 4 threads, set your differential to 2, your stitch length to 4 and presser foot pressure to light to moderate pressure. Hope that helps.
That said, it didn't produce a very gathered ruffle. I tried playing around with tensions ,etc but didn't write that stuff down
