And use normal overlocker thread of a matching colour on the lower looper and needle/needles.To flat lock you also need to change the tension.I have a Janome 4-thread overlocker and these are the tension settings-Stitch length 3;Needle thread tension 0 to 2;Upper looper thread tension 1 to 3; Lower looper thread tension 6 to9; You can remove the cutter, and if you can't, make sure your fabric is away from the cutting edge.
Some of the different types of materials able to be used in the looper are ornamental string,ribbon floss, metallic thread, or yarn.If you have a 4 thread overlocker and you don't want the seam to be too wide, you can remove the right needle.
I hope this helps. E-mail me if anyone has a problem.
Brenda Lea