When you click on the "Editor" button you will be able to choose a template to begin your new level with.
Those templates have all the important things already setup to help you get started and show you what a basic level setup looks like.
Make sure to save your level under a different name and save often in case of a crash. If you save your level make sure it is somewhere in the levels/ folder,
best practise is to create a subfolder which contains all the files necessary for your level, so you can easily send it to others to play it in multiplayer.
Real time editing:
The engine allows you to edit the level while people are playing on it in multiplayer, however some things you should not edit while doing that,
since they may cause a crash or other problems, those are in general the same things listed under "Things to not change in the world editor:"
What also causes a crash is if you save the level under a different name while people are playing on it, so if you want to edit levels in realtime and save them later
make sure to make a backup copy of the level and launch the server with the backup copy, which then you can savely overwrite, alternatively just save it under a different
name and kick the other players if you are done playing anyway.
How to make the level work with a certain mission type:
Since there are different mission types, you have to setup the levels to work with them, this is done by simply adding the mission type name into the mission files.
To do that there are two ways to do that:
One is through the world editor, where you go to the "theLevelInfo" object and scroll all the way down in the object inspector until you come to "dynamic fields"
where you see for example "levelName", there you have to add "MissionTypes" and set it to DM, TDM, PBDM or PBTDM or whatever gametypes your level is for.
The other way is to simply open the .mis file you created with a code/text editor and enter the MissionTypes manually.
While you are doing that you also can edit the "levelName" since this is very important, otherwise you will have dublicated names and you don't know which is which,
optionally you can enter some level descriptions there.
Remember that each mission type has some prerequisites, DM mission types need spawns for team0 and TDM mission types need spawns for team1 and team2, but the templates
already have some of each spawns as an example, you just have to move them, or delete them and create new ones in case you edit the level.
When you have done that your levels will show up in the level selection window.
Things to not change in the world editor:
There are certain tools you should not use in the world editor, since they will change the whole game and not just the level you are working on, those are:
-Material Editor (F4)
-Datablock Editor (F6)
Some editors have sub menus where you can make changes to the whole game, but are otherwise fine to use:
-Terrain Painer (F3) // Fine to use, but not edit the numbers of the materials after you click on "edit" since this will change the terrain textures of the whole game.
-Forest Editor (F8) // Painting forest is fine, but not edit the meshes in the "meshes" tab, since this will change them across the whole game.
This is just to advise you what not to use, if you just want to make a level.
The other functions are for advanced users, for example if you want to mod the game.
Best thing to do is to make a copy of the game you can mess around with and a clean copy you play with on the internet.