Morning Ruby, I'm not an overlocker expert, I've only ever used the one I've got now but I happen to be around at this time of the morning.
I wouldn't know which looper you need to thread first but this is a tip that I read in a book and it saved my sanity when I couldn't get my overlocker to stitch when I first had it.
It said that, with any machine, regardless of which looper the instructions say to thread first, the lower looper thread must always go over the upper looper thread before they go under the foot (a bit counter intuitive).
So, with my machine I thread the lower looper first as instructed then leave the thread end hanging down at the front of the machine. I then thread the upper looper and put the thread end under the foot then I take the first thread and put it under the foot - that way it goes over the top of the lower looper thread. If I don't do that I get a tangle when I start to stitch.
Just another aside while I'm here that may not be relevant, some machines, and mine is one, will not start a chain without a piece of fabric under the foot. Once I've done a short length of stitching on a scrap of fabric it will chain away merrily for ever more with or without fabric.