Think of one or two words that have deep significance to you and which you won't tire of having on your body. Think of words with similar letters that can be designed in such a way that they can be read both upside down and right side up --- for example, Family and Forever, Family and Honesty, Papa and Jedi, and names like Matthew or Nicole can all be made into ambigram tattoo designs.
Sketch out the word, name or two words on a piece of paper with a pencil. Look at websites and art books filled with samples of gothic, old English and Roman lettering styles for inspiration. Practice sketching the letters so that each letter looks like more than just itself. For example, draw an "i" that also looks like an "l" by making it long and underemphasizing the dot.
Try an online ambigram generator to get an idea of how to sketch out your ambigram design. Type in the first word, phrase or name that you want as well as a second if you want it to appear as a second word when viewed upside down. Examine the results, using them as a guide for your lettering. Purchase the design and take the stencil to a tattoo parlor of your choice or take your design to your tattoo artist and ask him to turn it into a stencil that he can use to place the design where you want it on your body in permanent ink.