Let's Split Flashing

(More information at qmk_firmware/layouts/community/ortho_3x10/wanleg/readme.md)

Make the QMK DFU .hex

make lets_split/rev2:wanleg:production dfu=qmk

Burning EEPROM settings and Firmware

Navigate to the directory with your .hex file and the eeprom-lefthand.eep and eeprom-righthand.eep files in it.
Burn Left Side With QMK DFU and Firmware
avrdude -b 19200 -c avrisp -p m32u4 -v -e -U lock:w:0x3F:m -U efuse:w:0xC3:m -U hfuse:w:0xD9:m -U lfuse:w:0x5E:m -U eeprom:w:eeprom-lefthand.eep -P comPORT -U flash:w:YOUR_production.hex:a

Burn Right Side With QMK DFU and Firmware
avrdude -b 19200 -c avrisp -p m32u4 -v -e -U lock:w:0x3F:m -U efuse:w:0xC3:m -U hfuse:w:0xD9:m -U lfuse:w:0x5E:m -U eeprom:w:eeprom-righthand.eep -P comPORT -U flash:w:YOUR_production.hex:a

Change comPORT to whatever port is used by the Arduino (e.g. com11 in Windows or /dev/ttyACM0 in Linux). Use Device Manager in Windows to find the port being used. Use ls /dev/tty* in Linux. Change YOUR_production.hex to whatever you've created in the previous step.

Using QMK DFU

Once QMK DFU is burned to your ProMicro, you can then flash subsequent hex files with make lets_split/rev2:<keymap>:dfu dfu=qmk
The dfu=qmk conditional will set BOOTLOADER = qmk-dfu instead of BOOTLOADER = caterina

Let's Split LEDs

In qmk_firmware/keyboards/lets_split/rev2/rev2.c, replace contents with

#include "lets_split.h"


#ifdef SSD1306OLED
void led_set_kb(uint8_t usb_led) {
    // put your keyboard LED indicator (ex: Caps Lock LED) toggling code here
    led_set_user(usb_led);
}
#endif

void matrix_init_kb(void) {

    // // green led on
    // DDRD |= (1<<5);
    // PORTD &= ~(1<<5);

    // // orange led on
    // DDRB |= (1<<0);
    // PORTB &= ~(1<<0);
	
	//turn off LEDs on ProMicro
   DDRD &= ~(1<<5);
   PORTD &= ~(1<<5);

   DDRB &= ~(1<<0);
   PORTB &= ~(1<<0);

	matrix_init_user();
};

to turn off LEDs