Hey!
I can only speak for myself, everybody got his/her own approach to stuff like this. First of all you gotta have a basic idea. This idea can come from anywhere. If you already have an idea for the theme of the level, try researching some real-life locations and draw inspiration for gameplay from there. If you don't have an idea, try hitting up http://leveldesigninspirationmachine.tumblr.com/
Once the basic idea is established and some reference pictures have been gathered, I like to start fairly simple, with a layout that makes sense for the type of theme. By fairly simple I mean only the main routes. Rotation routes can be added more easily along the go. This video might help you with starting the layout
Also: have you level playtested a lot! MapCore is hosting playtests two to three times a week. Feedback is crucial. You might feel something is too simple while other people might feel it's too complex. Don't get stuck in your own head. I also like to participate in other people's playtests to see what issues can occur so I can counter-act on them in my project. Plus it helps them too
Concerning modelling skills: It very much depends on what you are working on. If you want to make something fresh and new, modelling skills are essential, but you can create pretty good levels with stock assets too.
Hope this helps!
