Depending on if you've already customized your project or note, you can begin programming your GUI by using this as a base. I may recommend the other template I maintain as well if you want more features set up:
https://tofurocks.itch.io/renpy-gui-template
Otherwise, follow the instructions on this thread for just the code you want to use:
https://lemmasoft.renai.us/forums/viewtopic.php?f=51&t=48322