This mod allows you to modify the main menu using a json file, you can modify the content / position of pretty much everything the vanilla menu contains. In addition you can also add new stuff to your menu like a slideshow, web links or completely new sub menus.
After your first run with this mod the only difference you should notice is that there is an additional button added in your main menu. If you look in the config/CustomMainMenu folder you will notice a file called menu.json. In there you will see all elements the vanilla main menu has, modifying the content and pressing the refresh button ingame will allow you to instantly see what changes you made (You can remove the refresh button once you are happy with your result). Everything in there should be fairly self explanatory if you have ever worked with json files.