There are several tutorials for the different aspects of rust. I have learned more from other people but the tutorials are a nice start. There are alot of people to help you get going on starting a rust server.
To run a custom map you will check the custom launch parameters box in your control panel then you will add your custom map link there. it does not matter if you have anything in map seed or world size if you are using custom launch parameters. Here is an example.
+levelurl https://www.dropbox.com/custom.map?dl=1 +rcon.web 1
Also you need to have the oxide.ext.rustedit.dll installed into your RustDedicated_Data/Managed of your server and make sure oxide is installed and fully updated.
Here is the link to the rustedit.dll
I use magic description for my servers. I like the way it does it better. I notice if you set your description the set a variable then server.writecfg it overwrites it. That is the reason I use the plugin.
If you edit the file directly you need to restart the server. If you add them through console be sure to server.writecfg