Just like any other API. Add it as a reference in your C# client project. Check the FiveM docs for info on how to setup a C# resource.
Checkout the example menu provided in the download. It’ll show you exactly what all the features are. The rest is up to you.
Coming soon on these docs. Currently, use the source code as a guide (there’s comments in there for a reason). And use the example menu for some basic info. It’s designed similar to NativeUI’s structure, so if you’re familiar with NativeUI (C# version) then this shouldn’t be hard to switch to at all.