Although to be honest, you could probably do this easily with these steps.
1 – Write a clean RetroPie 3 beta 2 image to a SD card
2 – Expand card (raspi-config)
3 – Copy the romsets for lr-fba to your Pi (These are different to the romsets for pifba)
4 – Choose lr-fba as the default FBA emulator by pressing x when you start a game (you only have to do that once)
Your hotkey should already be set on the controller (select). Hold select and tap “x” on the joypad (or F1 on the keyboard) to bring up RGUI.