Datasheet

The bootloader on Pro Trinket is dual USB & FTDI "optiboot" so you can use either the USB port or the FTDI breakout

to upload. However, only the FTDI port allows you to use Serial debugging. So it's a bit of a tradeoff - USB is

inexpensive, built-in but requires a button press to start & there's no debugging, FTDI requires an extra cable but has

auto-reset and debug console.

Select the correct Board in IDE

Make sure you're selecting the FTDI-program type in the Boards menu!

Connect FTDI Friend or Cable

You'll need to solder 6-pin header onto the FTDI breakout first. Match the black wire and green wire with the BLK and

GRN markings on the PCB next to the FTDI header