Which section to start with when writing a research paper
There's no 'right' order. Starting where you feel able to do so is far better than not getting started. I usually write lots of sections concurrently, or start writing and work out what the sections are later.

On the other hand, it will make your life somewhat easier if you can work roughly in order of dependencies, so you don't have to keep changing what you wrote earlier. From example, it might be helpful to write out some of your notation before you start using it. The introduction will often come late in the process for this reason.