There are plenty of ways to start and finish a map, also for singleplayer.
An often successful approach is:
Start with the way of play first (also called "layout").
The map should contain the next couple things before you focus on the fancy stuff;
- Certain theme (gothic, urban, egypt, space, industrial, wild, etc; just pick one so far only for your first maps).
- Rough shape of how the map is going to look like (just slopes instead of stairs for example).
- Basic lighting (like sunlight only).
- Texturing that contains so far only walls, floors and ceilings textured individually.
- Gameplay elements (one playerstart, "carrot" (end point), monsters, weapons, health and armours, perhaps some spawnpoints, triggers, doors, etc).
You should have thought about a storyline roughly (or perhaps even before you started mapping; that's even better!).
Add some cubescripts for the doors or events. Perhaps borrowed from other maps; that's often not such a big deal (especially if you credit the original author).
If you've got a map which makes you able to start from and end while going trough some adventure(s); well done! You're map has a soul from now on (symbolically spoken)!
To be sure you've understand the limits of movement of Sauerbraten (or any other game on Cube Engine 2), I would recommend to take a quick peak on the Flow and Layout Guide. Don't read it by the word, but just look at the images and read the text generally.
After that, it's time to start working on details like a more interesting lighting, using multiple textures, models, sounds, etc. During this phase the map should get it's own identity.
For a more interesting lighting, I would recommend to read the Basic Lighting Guide. It's an easy to read guide which shows off commands that are noticeably and simply to use.
To understand pretty easy how to get a (map)model imported in your map, you could use the Model Prefab Guide, which is prefabricated configuration-file with explanation of all settings for models. If you might not understand certain commands , you can experiment with them or just keep them out of the .cfg-file for your model.
Last but least, check out maps that aren't in the official Sauerbraten, like from the Single Player Mapping Challenge: Tributional Page.