Plug in your Pro Micro and flash it to make sure it works. You absolutely want to make sure the Pro Micro works before soldering it to the board, as removing a defective one can be quite bothersome.
Get your VIA firmware from here.
QMK Toolbox (download the .exe)
If this is your first time flashing a Pro Micro, you will need to download QMK Toolbox as well as the firmware you’ll be flashing (both linked above, how generous). Run the program and allow it to install drivers when prompted. Next, you will Open your firmware file you have just downloaded. Your firmware file name may differ from the one in the following screenshot. Make sure the MCU field reads “atmega32u4” and the ‘Keyboard from qmk.fm’ field is left blank. Also, check the “Auto-Flash” box. Your QMK Toolbox window should look like this:
Inspect the anchor joints that secure the connector. If it looks like there isn’t enough solder, please add some solder to the joint on either side of the connector.
Now plug in your Pro Micro and locate the RST and GND pins.
You are going to form a connection between these two contacts using something like metal tweezers or a paperclip or a loose piece of wire. Very briefly make a connection between these two points and your QMK Toolbox should output a good number of lines. If you’ve successfully followed these steps, your QMK Toolbox window should look like this:
After you have it verified as working, place the short end of the pins into the PCB, and put the Pro Micro onto the long side of the pins with the components facing towards the board. This will leave the flat side facing upwards.
I then solder the four corners and ensure that the pins are perpendicular to the Pro Micro (straight up and down, basically.) After they are lined up, go ahead and solder all the pins, then trim the excess of the pins off as closely to the Pro Micro as you can. Then remove the Pro Micro from the PCB and place it to the side for now.