Gilding cookies, cakes, display pieces and other pastry items is super easy - it's just like painting, really. Here's a quick how-to that is part of my longer post about making gingerbread ornaments, which you might also want to check out.
1. You'll want to have a good assortment of paint brushes of several different widths. If you are painting over finely piped icing, you'll need a small, fine-tipped brush . The gold powder can be found at any pastry supply store. It isn't real gold and it's non-toxic, although real edible gold can be purchased for a higher price (inquire at your local pastry supply shop or online). If you're planning on gilding cookie ornaments, you'll also need piping tips in various smaller sizes (zero - four), piping bags and ribbon or string to tie and hang your ornaments with.
3. Here's a vial of gold powder up close. This little container runs about $6 in Manhattan, maybe cheaper elsewhere. Also, you can paint with this technique using any color of luster dust, it doesn't have to be gold.
6. Stir the liquid and powder together to make a paint. If it's too thick, you can add a few more drops of extract or liquor. If it's too thin, you can add more gold or wait a few minutes for some of the alcohol to evaporate.
There you have it - piece of cake, really. The only thing you have to be aware of is that it's important to take your time and be precise about guiding your brush, as it's hard to cover your tracks in the event of a major brush slip because gold in particular is unforgiving and impossible to wipe off without smearing it everywhere.
I hope you have as much fun painting shit silver, gold and other colors as I do! And don't forget to check out my accompanying cookie ornament how-to.