If you know where to go on the Internet, you can pick up an FTDI USB to Serial adapter for one dollar and sixty-seven cents, with free shipping worldwide. The chip on this board is an FTDI FT232RL ...
This is a 3D printed ‘clip’ for six pogo pins, designed to clip onto the end of an Adafruit Pro Trinket for FTDI programming. Before designing my own pogo pin adapter, I made Fiddy.
Add or Remove Programs entry for FTDI USB Serial Converter Drivers. This is a driver pack that allows USB ports to be used to connect to Serial bus devices with a USB - Serial adapter cable.