If you want to use white color only, you can use the barrel jack adapter delivered with the power supply to connect the “-“ output to the “white“ cable of the neon LED adapter and the “+” output to the “red” wire of the neon LED adapter. The cable has markers on it to see which is + and + on the other end too.
After you press the neon LED strip onto the printed snowflake you simply need to cut the strip on one of its markers shown on the side of the neon LED strip. The snowflake is sized to hold the strip 1 way around with not much overhangs of the LEDs as shown.