Summarize:

The three sisters need plenty of space to grow to their fullest potential. Ideally, the plot should be at least 10 by 10 feet (3.0 m × 3.0 m) large. The plot should not be near any tree, sheds, walls, or other shaded areas. A neutral pH is ideal for growing corn, squash, and beans. Contact your local extension office or gardening store for a soil pH testing kit. If you need to, amend the soil before planting to get it to the right pH. If you need to raise the pH, mix lime into the soil. If you need to lower it, add sulfur. These can be bought at garden stores and nurseries. Heirloom varieties are more traditional, and they grow well together. Non-heirloom varieties of beans, in particular, may grow too large and overwhelm the other plants if used.  Choose flint, dent, or flour corn varieties. While you can plant sweet corn, you will have to harvest it much earlier than the other plants. Choose pole or runner beans instead of bush beans. Dry beans, such as pinto or kidney beans, or green beans work well. Pumpkin or squash works well in this configuration. Winter squash and sugar pumpkins are ideal. Use your hands to push and pack dirt into mounds. Flatten out the top of each mound. Each mound should be 1 foot (0.30 m) high and 3–4 feet (0.91–1.22 m) wide. Keep each mound 3–4 feet (0.91–1.22 m) apart.  Mark the center of each mound with a stick. This will help you measure and find your mounds. When you plant the vegetables, corn will grow in the mound with beans around the corn and squash around the beans. Many people plant the three sisters because they grow well organically. To keep up with this practice, prepare the soil by applying an organic high-nitrogen fertilizer around the mound. You can use manure, fish emulsion, or—for a very traditional approach—fish scraps. If you decide that you don’t want an organic garden, you can use a basic 10-10-10 fertilizer.
Choose a large spot that receives at least 6 hours of full sunlight. Test the soil for a pH between 5.5 and 7. Select heirloom varieties of corn, beans, and squash. Create mounds for each plot of plants. Fertilize the soil with traditional or organic fertilizers.