A quality artist's brush
Goldleaf (real or dutch metal) can be applied to any surface, smooth, flat, carved, etc. For a perfectly smooth surface gesso is applied and and sanded. Always wear a mask while sanding gesso or sanding anything for that matter. Gesso is not necessary but to achieve a smooth surface it is a must especially on a large flat surface.
Some paint the surface traditional red, yellow-gold, blue, even black. This is a personal preference. I for one do not care for the red showing through the goldleaf preferring the soft tones of the yellow-gold instead. Try different combinations. Blue is especially nice under silverleaf.
Clean your hands to remove oils which will cause the leaf to stick. Gold leaf is very fragile and will crumble if not handled properly. Real gold leaf is especially fragile and more difficult to work with. If using real gold leaf you may want to invest in a gilder's tip, a flat brush made from squirrel hair around $75.00 or more. To use a gilder's tip you create electricity in the brush to lift the leaf and apply to the surface. This takes practice.
Once your surface is prepared (sanded, cleaned, wiped with a tack cloth, and / or gesso) apply the goldsize with the artist's brush. The size of the artist's brush will vary according to the size of the surface to be gilded. I would suggest you invest money in a quality brush, you would not want brush bristles to fall onto the surface. Quality or not, most brushes DO lose their bristles so before using vigorously tap the brush to rid it of any loose bristles beforehand.
If the surface to be gilded is large you may want to divide the area into more manageable space. Although goldsize takes a very long time to completely dry (I have doubts that it ever dries) you risk the tacky surface becoming contaminated by dust. After applying the goldsize you must wait until the adhesive is almost completely dry but just tacky enough for the goldleaf to adhere. this way you achieve a smoother finish. Goldsize could take several hours to dry, brands differ greatly.
Now you are ready to apply the goldleaf. If the leaf is patent leaf, usually the case, lift the leaf by the separating tissue and place face down onto the adhesive. With the tissue press into place, carefully lift the tissue and discard. With a soft cloth brush away the excess leaf. I keep a box handy to collect the loose leaf because you may reuse this leaf for smaller projects or repairs. Do not worry if the background color shows through, this is a desirable effect. With the same cloth you may polish the leaf.
If gilding a carved surface be prepared to use more goldleaf. you may need a stiff brush to press the leaf into crevices.
Real goldleaf needs no protection but composition will oxidize and must be sealed. Wait several days after gilding to give the adhesive time to dry then you may seal with another coat of goldsize or if using in combination with decoupage, you may apply polyuretane or varnish.