Firstly when where is talk about zodiac signs, it is assumed that reference is to sun signs commonly used in the west. But zodiac signs are also local to several cultures in the east. For example India and China have their set of zodiac signs. Both these culture also use 12 signs and the exact sign is determined by the date of birth of the individual. But these are based on complex lunar calendars.
Chinese zodiac signs consist of 12 animals. But the signs are designed in a very creative manner. Chinese zodiac sign is determined by the year of birth of the individual according to the Chinese lunar calendar. Each Chinese calendar is represented by a particular sign. Indian zodiac signs also consist of 12 signs but these signs have different names and designs than those used in the west. To determine the Indian zodiac sign it is important to know the exact time of birth as this system is based on the movements of the moon across the celestial.
Once you have decided upon the zodiac design you will be get tattooed, you can go the easy way and just do a Google search on it and get hundreds of generic designs and use one of them. The disadvantage of this approach is that your tattoo will be really commonplace and no-one will even care to give it a second look. For example if you want to get a tattoo of Libra, you can just take a design showing two unbalanced scales and you are done. If all you want is to get some dime-a-dozen tattoo this should do, but if you want to get a special tattoo which will be gawked at, wherever you go, you will need to do more work on your zodiac design.
Zodiac designs can be easily customized with some out of the box thinking. For example each zodiac sign is associated with a particular gem, so your design can be combined with a design of a gem, moreover each sign is associated with particular colors, so you can color tattoo based on that. Each zodiac sign is also associated with a particular earth element like fire, water, air, etc. You can use your imagination to combine your specific element with your zodiac sign to create a unique design.