A useful alternative to BYOND's built-in map editor, StrongDMM is a downloadable map editor for Windows/MacOS/Linux built to assist you with map creation and editing.
If you want to get started using it, you may consult a quick-start Guide to Mapping. It's been tooled to explain several concepts to absolute begineers, but there is still always more to learn. Although some of the advice was built on one specific flavor of Space Station 13 Code (/tg/), there should be enough to guide you through using it in the specified sections.
If you wish to skip the guide and want to play it out on your own, simply go to the page and scroll down until you find the download link pertaining to your Operating System. Run it and point it to your project's .DME (Environment File) for the tool to understand your project's parameters and generate the content needed to map. Then, you can create edit map files (.DMM) to your heart's content.