By myself and with my own intellect (almost – thanks to HEROESWORLD).
Those who knew everything all along (Xaker, for example:smile31:) could have shared the details. For everyone else.
But since no one is talking, I'll tell you.
Go into the data folder, then into data.pak/Maps. Look around. Here’s an example using Contested Island (Maps/Multiplayer/A1M3/A1m3). I simply extracted it from there. Created a1m3_1.zip/Maps/Multiplayer/A1M3/{all files here}. Renamed a1m3_1.zip to a1m3_1.h5m. It didn’t work. Then I “cut out” one of the folders named A1M3, resulting in a1m3_1.zip/Maps/Multiplayer/A1M3/{all files here}. Again, I changed the extension to h5m, put it in maps, and voilà! Now I can edit Contested Island however I want. Of course, it will be a separate map. And there are also some problems with the texts. More precisely, there are no texts (which isn't really a problem:)). You need to rewrite them (or find them somewhere in texts.pak and copy them from there).
Read more here: How to create Mods.
If something is wrong, please correct me (I only tested this practically).