Luckily there are lots of online sources that offer instructions for design, and many that use digital design and interactive software so you can print out your plans and bring them to a contractor. Check out DesignAPool.com or MyDreamPool.com. Be sure to consider price, your lot size, visibility in case you have kids, and extra features like waterfalls, slides, and vacuums.
You design it by making a list of your budget, what features you must have and what you can do without. Decide if it will be above or below ground. Also decide what kind of materials it will be made of and how many people will be using it. When you have the basic idea contact a pool builder and give him your ideas.