I serge with a Janome Compulock. I purchase 3 yards of fleece for my blankets and cut it in half. I use two layers making my blanket 1 1/2 yards long.
My cutting knife is lowered, you do not want to use the knife for this stitch.
I set my machine up for a 3 THREAD OVEREDGE STITCH
Right Needle removed
Left Needle is set at 3.0 and is threaded with serger thread
Upper Looper is set at 3.0 and is threaded with 300M wooly nylon
Lower Looper is set at 4.8 and is threaded with #10 crochet thread
Stitch Length is 4.0
Differential feed was set at 1.0
If your machine does not have the ability to use these settings, just set your particular machine up for the 3 THREAD OVEREDGE stitch and tho only real change I made was to the lower looper.
I set the lower looper to 4.8(tightening the stitch) so the thread would wrap the stitch to the back side of the blanket. Usually the lower looper and the upper looper thread meet in the middle at the edge of your material. I changed the setting so the lower looper pulled the thread off of the middle edge of the material and pulled it to more of the backside of the material. So now the front looper thread wrapped around the front of the blanket over the edge of the blanket and ended about 1/2 way down on the back of the blanket.