No, you don't add any water at all. You put the herbs in the canning jar and pour the 100 proof vodka over them. The amount of float, which is the vodka that floats on top once the herbs have settled is a matter of preference. I think what most people on the forum do is a 2/3 herb to 1/3 float but you can end up with whatever you want.
The most important thing when tincturing for the first time is to NOT put too much herb for the amount of vodka you use. For instance, both the Echinacea roots swell more than anything I've ever tinctured and if you aren't careful you'll end up with an impacted jar of herbs lol. That isn't really a problem though because you can add more vodka, it usually swells as much as its going to swell within about 24 hours. The problem is that you might have to end up making two jars of tincture instead of one ;-) Uny made a great post about just that kind of mishap when she made her first tinctures of Echinacea but I don't seem to be able to find it. I'll post it for you if I do, its really funny.
Tincturing is so empowering so have fun with it and please let us know how it goes.