Look through the Book of Kells and other Irish Illuminated Manuscripts to familiarize yourself with styles of knotwork and the symbolism behind them. Make note of the design that appeals to you both visually and symbolically.
Photocopy images from the books and print out others from Celtic design websites. Begin to compile a selection of images to take with you to your tattoo artist.
Talk to others with Celtic tattoos to find out where they got them. Ask if you can look at their work up close to get ideas for your own. If you can draw, carry a sketchbook with you and make a quick sketch of pieces you particularly like. The more visual reference material you can collect before your tattoo appointment, the better.
Take a trip to the tattoo shops in your area and look at artists' portfolios. Most artists who have been tattooing for any length of time will have Celtic designs in their books, as they are quite popular. Chat with the artists if possible to find one with whom you really connect.
Give your sketches and printed images to your tattoo artist and allow her to redraw the piece before tattooing it on you. Trust her to know how to amend your design so that it translates well to the area of the body you have chosen and your skin tone.