Steps

1. download this mod
2. add it as a dependency to your mod
3. require the gui-handler.lua file wherever you need it (local gui_handler = require("__JanSharpsGuiLibrary__/gui-handler.lua"))
4. require the basic-classes.lua file wherever you need it, although simply requiring it once is all you need. I would still recommend requiring it in every file which uses the basic classes (require("__JanSharpsGuiLibrary__/basic-classes.lua"))