Ok,...Also, what percentage of flavoring ought I to use in my juice?
Regarding flavor percentages, it varies a lot with what you are using. Some need 15% or more, others are disgustingly strong at 5%. First resource is the place where you bought it and/or the manufacturer's website. See if they provide a recommended percentage range, then start at the low end and work your way up. The problem with too much flavoring is that sometimes that will make it taste less like what it is supposed to. Hoosier's blog (see Danny's post above) has a good flavor troubleshooting guide that explains this. Danny's post is really comprehensive, btw, it will tell you almost everything you need to know, and the collection of links to suppliers will be helpful.
The other resource for recommendations is here, in the DIY forum. There is a spreadsheet around somewhere with some recommendations for a whole bunch of flavorings, and you can get advice for many flavors by posting a question that includes the name(s) and brand(s) of the flavoring in the title. That will get you the most responses from people that have used them. But even then you may need to experiment a bit to find what works best for you, since taste is very subjective and what works for one person might not work for someone else.