This mod manager is currently in Alpha, and is subject to make mod breaking changes, but is currently released for feedback. Also, this mod manager will be merged with cygrind once it's ready, and this repo will be archived.
First you will need to download an installation of UMM (an installation tutorial can be found here), and then you will need to create a new Class Library (.NET Framework) in Visual Studio (preferably using C# version 4.7.2). After that, reference UMM.dll, which should now be in ULTRAKILL\plugins\UMM. The Mod loader will look for a class that inherits UKMod, and has the attribute UKPlugin, like so.
Documentation for the API will can be found at the Wiki.