Applying gold or silver leaf can be really intimidating. It is expensive and you really don't want to mess up a perfectly good painting. Here are a few tips that I have picked up over the years:

1. Use super smooth paper. I like to use Yupo, or paper made of stone. 
   
2. Follow the directions on the adhesive. Wait until it's ready.
3. Use the softest brush you can find to brush away the extra leaf.
4. Don't apply leaf to a completed painting. Get about half way, and then apply. Then, if you mess it up, it's not so devastating.

Here's a video of me apply silver leaf to a small hare illustration.