How to use a Pro Micro as a Usb To Serial Adapter.  

First you have to burn the USBtoSerial firmware on the Pro Micro. For that you can use for example a UsbTiny or another Arduino (see You find the hex file on the right side under "Downloads"

 For USBtiny you upload the firmware with:

avrdude \
  -C<Arduino IDE Path>/hardware/tools/avr/etc/avrdude.conf \
  -v -patmega32u4 -cusbtiny \
  -Uflash:w:Arduino-usbserial.hex:i \

Now the Pro Micro is ready to be used as a USBToSerial. Howerver you need a 1k resistor and a 100n capicitor for the DTR (to reset the slave board).