Check out Hopoo's guide to the basics of the editor.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=651807566
This will teach you how to access and use the level editor in a step-by-step format.

Be sure, when saving your level, to manually add the .nc extension otherwise it won't work. There's also this guide that goes into more detail written by JAG and myself, though I'd advise you make a basic level first so you can get to grips with the editor.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=761140674
I hope this helps!

thanks