Gilding, as the use of gold and silver leaf is called, is traditionally a painstaking process that can take years to master. Using artificial leaf to create a convincing metal surface on a prop still requires a bit of skill and some practice, but a lifetime of apprenticeship isn't usually required.
First, the surface must be prepared. Because metal leaf is so fine, it tends to emphasize an object's irregularities, rather than hide them. The shininess of the finished product depends directly on how smooth and hard the surface is. Traditional gold leaf is applied over layers of gesso or clay which have been sanded to glassy smoothness. For stage use, the surface must at least be sanded smooth with 400 grit sandpaper, and sealed with shellac. The next step is to apply a coat of gilding size. This is a varnish or water-soluble adhesive which remains sticky for a while after it dries. The traditional varnish is more durable, but flammable, toxic and can be trickier to use; water-soluble size is much easier to manage. Other types of glue will not work for gilding.
When the size has dried enough (follow the instructions for the product you are using, but if it does not come off on your finger when you touch it, but is still sticky, it's probably ready), pieces of leaf are carefully pressed into it. This is harder to do than it sounds. Because the leaf is so thin, it tends not to tear where you want it to, and the tiniest bit of moisture or oil will bond it to your fingertips before it ever reaches the prop. I find it easiest to use a very sharp razorblade to cut each sheet of leaf into manageable pieces, and a soft sable brush to lift and apply each one. If you find it is sticking to your fingers, a dusting of talcum powder on your hands will usually solve the problem. Overlap the pieces as much as necessary to cover the entire surface, being careful to fill any gaps or tears that appear in hollows and crevices.
Once the surface is covered, it should be rubbed lightly with a piece of
silk or soft tissue paper to smooth down any loose bits and rub away the
overlapping edges. This leaves a warm, satiny surface, shinier than paint but
still quite soft-looking. If you need a harder look, it has to be burnished to
smooth down the surface of the leaf and make it more reflective. The best tool
for this is a smooth agate or bone polisher, though you can get a pretty good
shine by rubbing it with a piece of hard plastic or smooth hardwood. If you've
used a water-soluble size, let it dry overnight to make sure it's good and dry
before burnishing .