Prepare the surface to be gilded by removing any corrosion and repairing any surface defects. Sand the surface with 220 grit paper to make it as smooth and even as possible. Remove all dust with a tack cloth.
Lay down drop cloths and mask off any areas that will not be primed and gilded with painter's tape or similar material. Liquid frisket may come in handy for intricate interior objects like small picture frames.
Prime the surface to be gilded with one or two coats of quality primer suited for the type of material being gilded. A gilding supplier will have appropriate primers for gold leaf and typically in red, gray and ochre. Each can offer a different look to the finished gilding.
Apply a "bole" or base coat color if necessary, but note that this step can usually be skipped when using modern primers tinted to the color of choice.
Select the gold leaf to use. Gold leaf is sold in "books" of 25 sheets or in rolls. Books can be either loose or patented sheets. Rolls are always patented. Patented means the leaf is attached loosely to a backing and this can make it easier to handle and should always be considered when working in outdoor and breezy conditions. Gold leaf is also sold in karat weights and as alloy mixes. 24k is the most pure form but the most expensive.
Size the surface with adhesive sizing. Sizing provides a "tacky" surface the leaf can stick to and comes in a variety of both water and oil based formulas. Choose from a gilding supplier depending on the look you are trying to achieve and apply as thin and evenly as possible with a high quality brush.
Test for tack. Some sizing will remain tacky when completely dry but most will have a period between being wet and completely dry that is the best time to apply the gold leaf.
Pass a gilder's tip lightly through your hair to charge it with static electricity. Gilder's tips are special short brushes in cardboard handles used for handling loose gold leaf. Hold a book of leaf in one hand and gently touch the gilder's tip to the top sheet of leaf. Transfer to the sized object to be gilded. Burnish in place with a gilder's squirrel mop or goat hair brush.
When using patented leaf, place the leaf face down with the backing attached. Burnish through the backing with a gilder's squirrel mop or goat hair brush. Gently remove the backing. Burnish the leaf itself again as needed.
Apply one or two coats of an acrylic clear coat if desired or necessary using a soft, high quality brush or sprayer.