In your SH3 folder is "SH3MissionEditor.exe", which is the easiest way to add or alter content. SH3 uses 3 big files, "Campaign_LND.mis", which holds all the "land" units, such as shore guns, AA guns, AirBases, NavalBases, etc. Then there is "Campaign_RND.mis" which holds all of the "random" groups - which can contain units or groups of units. Those are called Random Generated Groups (RGG). Third is the "Campaign_SCR.mis", which holds all of the "scripted" units and groups in the game. The size and date range make the files appear to be overly complex, but it's about like any other puzzle that has a lot of pieces that have to fit together. To explain it would take a thread unto itself, which there are some on the site here, just above your post here. Read through some of those. If none of the links have the v17 of the SH3 Mission Editor manual, then use this one:
MissionEditorManual.7z
... which has a folder "MissionEditorManual" and inside that is the "[ENG] Mission Editor V17.pdf" file. That is (imo) probably the most thorough discussion of the Mission Editor there is. Start with a Single Player Mission, and build from there. There is a "video player" in the ME (Mission Editor) that can be used to see how your scenario might play out in the game. In the link for the MissionEditorManual above, on page 56, they discuss the use of that a bit under "Simulator controls". Very useful.
The main thing to do is to create yourself a mini-mod to do your testing in before you go messing with the main files. So make yourself a folder in the MODS folder of your game, and name it whatever you want, like "CampaignAdds" or whatever. Inside of that make a folder named "Data". Inside of that, make one named "Campaigns" (note the "s" for plural) and inside of that, make another folder named "Campaign" (no "S") - these are for the Career Campmaign files. Also under the "Data" folder, you can create a "SingleMissions" folder, and under that either an "English" or a "German" folder, dependent upon your language used. Under that you would then create a folder to hold each of your mission files, but they have to be named
~EXACTLY~ like the mission file name. Those folders are where you will "Save" your work to, and then activate through JSGME for testing.
Do some Single Mission building experiments. Just be sure to place your submarine in the scenario, and mark it as Player Controlled (way too easy to miss). Use the "Mission" menu, and "Parameters" to give the mission a name that displays on the menu (does not have to match the file and folder names). Put something in for the briefing there also. Be sure that what you name the mission when you go to save it, that you put it inside of a folder of the same exact name, else it won't show on the menu in the game. Let someone here know when you think you have something you might want to add to the campaign, and we'll talk about the use of "Merge", and its good uses, and dangers. You'll use it first to check the placement of what you're adding, then again to bring your edits into the game...
