December 31st will be your opportunity to easily locate and observe Neptune with a telescope as it will be within 1/3 degree of Mars low in the western part of the Sky. Mars will present a reddish magnitude 1 disk while Neptune will be much smaller, essentially a dimmer magnitude 7.8 dot. Large telescopes should reveal the blueish hue of Neptune when placed slightly out of focus.
In the image above I’ve marked magnitude 7.9 star just outside the 1 degree circle to assist in the orientation.
However don’t wait too late in the evening, best may be shortly after 7pm once the Moon is below the horizon. Starting from the horizon you’ll able to easily locate bright Venus and about 10 degrees above will be Mars and Neptune. Bright stars Fomalhaut and Altair will be located east and west along the horizon.