The following website has a bit of a guide on trimmining nails, including a visual.
The only problem is, if you are working with a bird that has dark toe nails... or an extremely long quick. If the nails are dark, you just trim a small amount at a time. If the quick is long, same idea, although you may want to trim at the same length until you've found that the quick has receded.
Here's another article
Toenail Tips for Parrot Keepers
It's always easier for me to trim Charlie's nails using a dremel vs some sort of shiny clippers, which I don't quite understand why, since the same dremel gets used on his beak, and if that thing is on, he wants nothing to do with me! I should probably get an emery board for the birds.
Casey Bourke Pistachio Tomi Girl Charlie Faye Jayde Sylphie (Foster)