When you’re ready to shoot, kill the lights and start your bulb exposure. Turn on your LEDs and start a big spin from the bottom. Using the string, slowly pull the LED’s up with them still spinning, as you pull the string, the LED side gets shorter and the arc gets smaller, creating a spiral like tree shaped effect. Once the LED gets to the top and the spin is really small you can stop the exposure. If the exposure of the tree is a little dull, simply up your ISO until you get the exposure you require. If the spinning isn't working very well, you can also try the same pull method with swinging the lights from side to side.
Adding tree decorations
If you want to light paint additional details such as decorations, simply turn the LED off at the end of the spiral and add your details in. Wearing black clothes will help keep you from appearing in the shot.