The most recent version of the standard bigtreetech TFT firmware has built in support for RepRapFirmware. The pre-built images have this enabled by default.
Connection to the board
Use pins +5V, GND, TX and RX on the “screen/tft/aux” header. These should be connected to +5V, GND, TX and RX on the TFT, making sure that TX and RX are swapped.
The specific screen pages for each board have more information.
Loading the firmware
There is now an RRF config.ini
It needs to be renamed from config_rrf.ini to config.ini for flashing of the firmware.
Add the following line to your config.g to enable the screen
M575 P1 S1 B57600