Search through books, magazines and on the Internet to get some ideas for your design. You may already have an idea of what you'd like, but looking at pictures of other tattoos will help you to "fine tune" your design.
Visit your tattoo artist and speak to him about your design ideas. A professional will be able to give you good advice and pointers, and can even complete the design and produce some art work for you to have a look at. If you don't have a tattoo artist in mind, then make sure you find one who has a good reputation, a portfolio of their work for you to see, and preferably, someone who comes highly recommended.
Draw you design onto paper. Do not worry about size at this stage, the idea is just to be able to study your design and ensure you are happy with it. When you have finished your design, hide it away for a couple of days and then go back to it. This allows you to look at the design with a fresh pair of eyes and decide if there are any adjustments you would like to make. Use the link in the Resources section to insert a picture of your design onto a virtual body. This will give a really clear picture of what your finished tattoo will look like.
Take the design to your chosen tattoo artist. He will be able to ensure it is reproduced at the correct size to fit in exactly the right place on your back. If drawing isn't your thing, then a simple scribble should be enough to give the tattooist a good idea of what you're thinking. Take as much information as possible, pictures from magazines for example, and the tattooist will be able to put some art work together. Tweak the design until it is exactly what you had envisioned.