Don't.

Remember our layer system is the result of lots of work on lots of simultaneous problems. As such, it is hard to improve but easy to break. Trust me. Further, layer management is far and away the most unpleasant standards task in the whole operation. You don't want to get involved.

OK, sometimes you have to. Maybe you really need a new layer. Maybe I told you you need one. If you're going to do it, you'd better do it right.

Select an existing LC that is similar to the one you need to make. Click 'New'. (The other one. Lower left.) Put in the new name. Make changes to the showing and hiding of layers on the right. Click 'Update' at the bottom. OK and save.

You will probably need to create views for the new LC.

See also:
Creating Layers