Solder the female pin headers for the trinket on the bottom side of the board.

Solder the MQ135 gas sensor on the bottom side of the board. It does not matter which way you solder it in! Note: The footprint is a bit small so you might not be able to push the component completely in and you might have to bend the legs a bit!

Turn the board around once more. Solder the male pin header for the I2C, serial and buzzer connectors to the top side if needed. Note: Solder some wires as shown in the picture from the trinkets ground pin to the ground pin of the serial connector and the heating element of the gas sensor!

Download the SniffingTrinket firmware and upload it via the ArduinoIDE. Have fun!