First I would start by getting practised in graphic design / UI design tools such as Photoshop, Illustrator, Affinity... There are many amazing tutorials around the internet on how to get started. Start by looking at what you like and dislike about the current Marble Blast menus and go from there.
Next, you will need to download the official GarageGames Marble Blast Gold GUI and C scripts from this website, look for mbg-script-sources on the Extras page. Marble Blast uses both .cs and .gui files, these are compiled and loaded at launch into .cs.dso and .gui.dso files. In Marble Blast by default there are the compiled version however to make new / modify these, you will need to replace them with the decompiled scripts that you downloaded.
You will find most GUI scripts in /marble/client/ui. You will need these files for the main menu (Play, Help, Options, Quit menu); MainMenuGui.gui and /client/ui/home for the photos.
From there, start writing custom parts, change numbers and see what happens!