Vial support
RMK uses vial as the default keymap editor. By using vial, you can change your keymapping at real-time, no more programming/flashing is needed.
To persistently save your keymap data, RMK will use the last two sectors of your microcontroller's internal flash. See storage. If you don't have enough flash for saving keymaps, changing in vial will lose after keyboard reboot.
Port vial
To use vial in RMK, a keyboard definition file named vial.json is necessary. Vial has a very detailed documentation for how to generate this JSON file: https://get.vial.today/docs/porting-to-via.html. One note for generating vial.json is that you have to use same layout definition of internal keymap of RMK, defined in src/keymap.rs or keyboard.toml.
In vial.json, you define the keyboard layout, specifying which keys are in which position. In src/keymap.rs or keyboard.toml, you define the keymap, meaning which symbol will be printed by the press of which button.
This is the default keymap, which you can change using the vial app (or the web app). Unless you set clear_storage = true (see [storage]), these changes will persist when you reset your keyboard.
After getting your vial.json, just place it at the root of RMK firmware project, and that's all. RMK will do all the rest work for you.