The thing that worries me most about people who are new mixing their own juice is that they have proper volumetric equipment to measure everything out accurately. If you don't have that, I would say don't start mixing. If you do, use the calculators and Ryedan's advice on where to look.
Psshh. A little bit of this, a little bit of that, a few drops of those and a few drops of these.
Seriously, good recommendation. Being able to measure reliably makes all the difference in the world. This is especially true if you have that Eureka! moment with a liquid you mixed and you want to replicate the success.