Set up your desired controller layout by using the dropdowns to select which buttons on a standard Xbox-gamepad layout should map to the given Dreamcast button.
Clicking the 'Download Mapping' button will trigger the download of a small binary file for the desired controller layout.
While Pico2Maple is in BOOTSEL mode, use picotool to load the mapping onto the device using the following command:
picotool load "pico2maple-mapping.bin" -o 0x103FC000