When booking your Celebrant I recommend hiring someone you can relate to, someone that fits in with you and your partners personalities and energy. This is important so your ceremony will be as reflective and indicative of you as a couple.
Some people are more conservative so choose a celebrant that will stick to a more traditional approach. I personally looked for someone who had the perfect mix of professionalism, sense of humor, style and quirkiness. I wanted someone who radiated joy and genuineness.
Sometime you will choose a celebrant purely by looking at someones website or Facebook page, sometime's you will need to meet the celebrant in person, others might just grab the first person who pops up on google or someone you have seen in action before at a friends wedding.
Whatever floats your boat! Your Celebrant is like a Mascot for your big day!
Go with your Gut!