You need a minimum of 52 white polystyrene balls to carry out this project, but it is advisable to have at least 10 spare balls for any that might be damaged. Separate your balls into piles of 18, 16, five, five, four and four, which represents the six molecules within DNA: pentose, phosphate, cytosine, guanine, adenine and thymine, respectively. Choose a paint color for each of the molecules -- it does not matter which color you choose, as long as each molecule is easily distinguished from the others. Paint your groups of balls with your chosen color before leaving them to dry in a warm, dry environment.
DNA molecules occur within your double helix in specific pairings and orders. For example, you can only join cytosine and guanine together while adenine and thymine are exclusive to one another. Furthermore, there are a total of 17 layers in your DNA model, with layers alternating between two and four molecules. Where there are four molecules, such as the bottom layer of your model, there is a pentose molecule on each end. Where there are only two molecules, such as the second layer of your model, they are both phosphate molecules and occur on the end of the layer. Cytosine-guanine pairings and adenine-thymine pairings alternate, so while your bottom layer consists of cytosine-guanine, the next four-molecule layer --- the third layer --- is adenine-thymine. Furthermore, the molecule pairings themselves alternate and change order --- for example, the fifth layer is thymine-adenine, and the seventh layer is guanine-cytosine. Join all of your molecules together using double-ended toothpicks by pushing them into the center of the polystyrene balls.
To get the best impression of how your molecules join together and are laid out when constructing a DNA model, print out a DNA map, poster or model photo. Follow the molecule construction of the models displayed. Make your model simple by first laying out all of the painted polystyrene balls in the correct order, starting with a bottom layer of pentose-cytosine-guanine-pentose, and a second layer of phosphate-phosphate, which should only ever be attached to the preceding and following pentose molecules. Lay out your third layer as pentose-adenine-thymine-pentose. Continue until you have laid out 17 complete layers that match the order on your DNA map.
Your model also requires a wooden stand in the middle. Find a stand with a fairly heavy, solid wooden base with a center column or pole protruding up at least 12 inches. Use glue, and twist the polystyrene balls in your experiment, where necessary, to introduce the double-helix DNA shape, which Monster Guide advises should start when you are four layers from the center of the model. Allow your model to dry completely before moving it to present to your fellow middle school students at a science presentation or fair.