This is a short tutorial on how to mod Interplanetary. It's not very difficult and you're ready to go in just a few minutes.
For the tutorial I'll be using Windows 10 and Open Office Calc. Of course you also need the game itself.

First we have to find the Interplanetary game folder. Therefore, open Steam and go to your library. Select Interplanetary, open the context menu and click properties.
Next you switch to Local Files and Search Local Files.

The folder should now open.

Next, navigate to the folder .../Interplanetary/StreamingAssets/Data and find the ModBase.zip folder. It contains a copy of each of the binary (.bin) files in the /Data folder. The difference is that the copies are editable textfiles. Never try to change the binary files themselves. Extract the ModBase.zip into a new folder. The textfiles hold all the lists and values you are able to edit. Of course you could just open the textfiles and edit them, which would look somewhat like this (Windows 10 Editor):


But as the files represent tables you could also use a spreadsheet application like Microsoft Excel or Apache Open Office Calc to edit them.
Right click one of the textfiles and select Open with... and select your prefered program. While opening it, your program might wonder how to import the .txt file. Using Open Office, this window pops up:
Make sure to turn all separator options off, except for Tabulator, which is used to determine columns.
Hit OK and you're good to go:

You can now change all the textfiles as you wish, which includes changing values of existing objects or creating entirely new ones. The tables are pretty self-explanatory.
Once you finished modifying your files make sure to save them, then copy and paste them directly into your /StreamingAssets/Data folder. If you didn't change all the files you only need to copy the ones you edited. While saving, watch out that the format stays the same (textfile).

Start your game, and in the options menu go to misc. Enable the custom files checkbox. Interplanetary will now overwrite the binary files with the textfiles it finds in the /StreamingAssets/Data folder.
To test your mod, start a new game. Loading an old game doesn't work as the old settings are used. Also make sure to close your editor or spreadsheet application, so that Interplanetary is able to load the files correctly. Mods should also work in Multiplayer games, but only in custom game lobby. For random matches the normal files are used.

There's a ton of different reasons for why your mod couldn't work as intended. Here are some possible fixes:

Game doesn't load up to main menu or has errors while the `Custom Data Files` option is turned off: This *probably* means that some game files have been damaged. Redownload the game and it should be fixed.

Weird values ingame, techtree doesn't show up, etc.: Your mod seems to cause issues. Make sure that your editor is closed. If your country normally uses `,` as decimal mark make sure that you use `.` instead. Sometimes your editor will change this automatically (!).
If you edited multiple files test one after another to locate the broken file.

No changes visible in game: Make sure you saved your work in the editor and closed it, have you also copied the new files in the /StreamingAssets/Data folder? Is the `Custom Data Files` option turned on? Did you start an entirely new game so the mod files can load up?

That's it for this tutorial. Of course this is only the beginning and not a complete list of all the possibilities to mod Interplanetary.
Please also have a look at the readme which is located on the /StreamingAssets/Data folder, I learnt most of the stuff from there.

If you somehow manage to mess up your game (like I did), remember that you can always redownload it. Don't worry, your savegames are stored in a different location and won't get removed (User/Documents/TJR/Interplanetary/save).