This is a great technique from the Malden Mills classroom page on how to flatlock fleece on a serger; it's wonderful for fleece socks and leggings/long underwear where seams could rub and chafe and cause blisters. The serger does NOT need the built-in flatlock stitch using this method.
" Flatlock Seam
Use a three thread serge stitch with no tension in the needle.
Join the two pieces together
Pull seam flat "
I've used this quite a bit on my White 1500 serger (3/4 thread) and it works very nicely!!