To grow square carrots, the soil is critical. Care must be taken in soil prep prior to planting. Ideal conditions require a very fine consistent texture free of rocks and debris and should drain well. Compacted rocky soil will interfere with the molding process. 
Compost and sand is a great way to loosen your soil and add the nutrients that your plants will need to grow strong and square. A complete guide to soil prep is included in your Square Carrot Kit.