Summarize:

For this version of the project, you will need small styrofoam balls, a needle and thread, paint, and toothpicks. Choose 6 different colors to represent the sugar and phosphate groups, and the 4 nitrogenous bases. They can be any 6 colors of your choice. You will need to paint 16 sugar balls, 14 phosphate balls, and 4 different colors for each of the nitrogenous bases (cytosine, guanine, thymine, and adenine). You could choose to have one of the colors be white, so that you don't have to paint some of the styrofoam. This might be easiest for the sugar balls, as it will greatly reduce your total amount of work. Once the paint has dried, designate 1 color for each of the nitrogenous bases, and then pair them with their matching base. Cytosine always goes with guanine, and thymine always goes with adenine. The order of the colors does not matter, as long as they are in the correct pairs. Stick a toothpick between each of the pairs, leaving a little extra space at the sharp ends of the toothpicks. Using the needle and string, cut a piece that is long enough to go the length of 15 styrofoam balls. Tie a knot at one end of the string, and thread the needle on the other. Line up the styrofoam sugar and phosphate balls, so that they alternate in sets of 15. There should be more sugar balls than phosphate balls. Make sure that the 2 strands of sugar and phosphate are in the same order, so that they line up when placed next to each other. Thread through the centers of each alternating string of styrofoam sugar and phosphate balls. Tie the string off at the end of each strand, to prevent the balls from sliding off. Take the toothpicks with your pairs of nitrogenous bases, and stick the sharp end to the matching sugar balls on each long strand. Only attach the pairs to the styrofoam balls representing sugar, as this is how DNA is attached in real life. Make sure that enough of the toothpick is attached to the strands that the pairs of bases will not fall off easily. Once all the pairs of toothpick bases have been attached to the sugar, twist the double strands in a counter-clockwise direction to mimic the appearance of a true double helix. Your model is now complete!
Gather your supplies. Paint your styrofoam balls. Pair off the nitrogenous bases. Make the double helix. Attach the nitrogenous bases to the double helix strands. Twist the double helix.