The official Donut Team docs have a page for it, however most people don't know how to access it.
Disclaimer: As the mod launcher evolves, please expect things to change. I will do my best to keep this thread updated but don't take it as done.
Setting it up:
1) Make sure your mod launcher comes from this page & not from another source.
2) First thing is locating your game, click on the [Open...] button & then Launcher Settings & go to the Game tab. There, you should browse your game (Simpsons.exe)
3) Logging in with your account: you should log in with your Donut Team account, there is no reason not to. To do that, you should click the [Account...] button & log in as the launcher tells you to.
4) To add a new mod you'll have two options: Either click [Open...] then scroll down to Mods (picture below) or right-click the Mod Launcher's icon and go to Open File Location... then locate your Mods folder. NOTE: Remember to click the Reload button everytime you add new mods.
This is pretty much it with setting it up!
2) First thing is locating your game, click on the [Open...] button & then Launcher Settings & go to the Game tab. There, you should browse your game (Simpsons.exe)



Using Mods/Hacks:
Mods: You can find them under the General or Unreleased tabs.
Hacks/Settings: You can find them under the Settings or Developer tabs.
1) To use a mod you must (obviously) check the box next to it.
You can also check out it's settings by right-clicking it & going to Settings or clicking Mod Settings right under the About box
Right next to the Mod Settings button you have a checkbox to favorite the mod.
2) Categorizing mods is also a thing! You can click the list under the General tab to reveal the following categories:
The Author category allows you to search a mod by selecting an author from a selected list:
3) Compiling/Decompiling:
You can compile your own mods if they're still within a folder & not a lmlm archive. To accomplish that, you right-click on your mod & go on the Compile button.
You also have the option to decompile certain mods (depends whether the author allowed it or not)
If you have multiple mods activated, you can disactivate them by clicking Disable All
Same stuff applies to Hacks/settings, very few things such as the categories list differ however.
Hacks/Settings: You can find them under the Settings or Developer tabs.
1) To use a mod you must (obviously) check the box next to it.






You can compile your own mods if they're still within a folder & not a lmlm archive. To accomplish that, you right-click on your mod & go on the Compile button.



Languages:
To add a language, right-click the Mod Launcher's icon and go to Open File Location... and locate the Languages folder (if not, make one).
Here is a link you can use & grab some preset languages from. Download at least one of them (.xml file) and paste it inside the Languages folder.
To apply it, reload your launcher, then click on [Open...], Launcher Settings & go to the Launcher tab. There you can set up your language.
Note: most (if not all) of the languages at the moment are user-made, so expect them to be either incomplete or incorrect on some cases (although not necessarily).
Here is a link you can use & grab some preset languages from. Download at least one of them (.xml file) and paste it inside the Languages folder.
To apply it, reload your launcher, then click on [Open...], Launcher Settings & go to the Launcher tab. There you can set up your language.

Crashes:
Crashes are going to be a big part of your life if you're staying for long on Donut Team. That is why, the launcher has a spot for them!
To access crash dumps, you should click [Open...] and go down to Crashes.
You can read these with @Josh / Proddy and @EnAppelsin 's Crash Analyzer.
To access crash dumps, you should click [Open...] and go down to Crashes.

Miscellaneous:
Screen Resolution & Running mode:
With the mod launcher, you can set up your resolution & running mode. You do that by changing the resolution from the list in the image and next to it, you have the option to toggle on/off the Windowed mode.
Mod Console:
You can enable the console under the Developer tab as a hack. It allows you to read logs from your mods, hacks & the game.
Debugging:
Similarly to the console, you can enable the following hacks under the developer tab: Debug Checks, Debug Test, Debug Text.
These three allow you to use a debug menu in-game for your mods.

With the mod launcher, you can set up your resolution & running mode. You do that by changing the resolution from the list in the image and next to it, you have the option to toggle on/off the Windowed mode.

You can enable the console under the Developer tab as a hack. It allows you to read logs from your mods, hacks & the game.

Similarly to the console, you can enable the following hacks under the developer tab: Debug Checks, Debug Test, Debug Text.
These three allow you to use a debug menu in-game for your mods.

I hope this post helps you get used to the launcher & understand how it works!