Take the black (-) wire of the sensor and solder it onto the GND of Trinket on the reverse side of the board. Now gather all the negative wires (white), and twist the ends together until they form a single wire. It helps to rubberband the other wires out of the way. Tin the end of the white wires.
Solder the end of the white wires onto the GND of Trinket. Having problems? You can always cut a short piece of white wire and solder it to the GND on Trinket and then solder the 4 white LED wires to that. Just make sure to apply electrical tape or heat shrink tubing at the junction.
Continue to solder the rest of the wires to their correct pins on Trinket. Make sure not to trim the length of the wires on the sensor. You may need to adjust the height later once you hang the mistletoe, and it is better to keep it longer rather than shorter.