You can do it either dry or with water with a very tiny amout of water. I prefer doing stripes dry. Simply because Ive done it a few times before & I am more comfrotable doing it this way.
Although for a first timer doing it wet will allow you to have a second chance because the sticker will not stick straight away.
Just be aware This job can be difficult to do properly & if you dont think you can do it get a local signwriter to do it for you. So many times Ive sold stickers to people who wanted to do it themselves & come back later to order another sticker & wanted us to put it on.
If you do it wet you can use a good quality Dishwashing detergent & put a small amount (like a couple of drops just to make it a little soapy, too mutch & the sticker will not stick for a long time) in a spray bottle.
One other thing is make sure the car is very clean first. Also clean the area that you are going to put the sticker with Isopropyl Alcohol or Wax & grease remover first to make sure there is no wax or contaminants that will affect the adhesion of the sticker.
Follow this video link & it will give you an idea on how to do it wet
http://www.youtube.com/watch?v=JKHmyhm7Ubo
Hope this helps, Good luck & dont forget to show us some pics of the finished result.