For best results apply a low luster oil-based base coat. Traditional basecoat colors are red (Chinese lacquer), yellow (French), or black (Japan). Your basecoat will give the gold leaf a rich look, as well as provide visual unity.
Apply a thin layer of gold leaf adhesive. The best adhesives are oil based, and come in quick dry/slow dry formulas. The quick dry will be ready for leaf w/in an hour or two, depending upon conditions, while the slow dry usually dries to a tack w/in 12 hours. Quick dry works well with synthetic leaf, and is a good all purpose, small project product. The slow dry is appropriate for use with more expensive "real" leaf. Allow adhesive to dry to a tack, apply gold leaf.
Applying gold leaf is very easy if you use wax paper. Cut pieces of wax paper a bit larger than the leaf. Lay one piece of paper over one piece of leaf. "iron" over the wax paper with your hand and the leaf will stick to the paper, then gently lay the leaf on the adhesive surface.
Let dry, and burnish gently with a soft cloth.
Sealing recommended over gold leaf- the same oil based adhesive sizing works well for a topcoat, or shellac, polyurethane.
Bear in mind that most commercially available gold leaf is what is referred to as "composite leaf," that is, it is not necessarily pure gold. A lot of these contain copper or brass, which will tarnish over time. Not before crit, to be sure, but certainly after a few years. Only gold leaf that is labelled "22k pure gold" or something like that is actual gold, which will never tarnish.
Only apply sizing (adhesive) to a small area at a time, because eventually it will dry out and loose its tack. It is ready when it feels like scotch tape.
If you rub your (clean, dry) leafing paintbrush on your hair, you will have enough static to pick up a sheet of leaf.