In order to make the Trinket board so small, the serial to USB conversion is left out from the design. On a regular Arduino board, this conversion is handled by another Atmega chip, and on older versions this was done by an FTDI (Future Technology Devices International) chip. The FTDI chips are still around and you can buy these as standalone breakout boards as shown in Figure 3.8 to the left of the Trinket board:

Figure 3.8: The FTDI serial to USB converter and the Trinket board

Normally, you solder male pins to the end of the Trinket board that connects to the FTDI converter, but in this case we want to keep the Trinket board as flat as possible and we don't want sharp pins on the inside of the glasses that might hurt your eyes. So the trick is to just attach the male pin headers to the FTDI converter and hold it in place while programming the Trinket board. Once in a while there will be glitches in the connection and the upload will fail. This is probably due to...