After getting your background sealed and shellaced, the next step is to apply oil size. Harder than it sounds... It is a kind of tacky varnish, with the number next to it (3,6,12,16 (which is much rarer) and so on) referring to the number of hours it will stay 'open' i.e. in which time the gold can be applied. It must be put on slowly, but not too slowly, carefully and with as few streaks as humanly possible. This is VERY tricky when it is not a simple straight or flat area, such as around the outline of a curly-headed angel with wings! When in doubt, take the size closer to the edge and you can either a) scrape the gold gently back later on or b) (and perhaps safer) paint over the gold in one or two tiny areas, as with oil gilding there are always areas which need to be readdressed. And I had that from a very experienced, talented furniture restorer and gilding teacher, so I'm not making excuses... well not many ;-)
To apply the gold, you must wait until the size is only just tacky and squeaks when you very, very, VERY lightly drag your finger knuckle along an inconspicuous point. Don't use a fingertip, unless you want everyone to be able to identify the gilder in years to come... When it squeaks and doesn't mark you are ready to gild. Although I used six hour size, it was actually ready to go a bit earlier than that - you can almost see the difference and if you have the luxury of a spare piece of wood that you have prepared, I'd make a sort of 'gilding sample' so you don't necessarily have to squeak the actual board. The sheet gold used is pressed into pieces of tissue paper, rather than loose leaf which I prefer as it gives an absolutely smooth finish - the imprint of the tissue means it will never be entirely mirror-like. Plus sometimes the sheets aren't stuck in very well, so if in doubt, gently compress the book before attempting to use the sheets. Trial and error here I'm afraid, mostly error on my part... You can either use a whole sheet, if it is a large flat area, or if you are gilding over borders or around a complicated area, use scissors (yes, but good quality ones with un-nicked blades) to cut up the sheet. You then apply the gold straight on, no brushes required and lightly press down on the back of the paper. I will admit right now that most of my gold came off the sheets (thank you unnamed supplier!) and I had no real idea what I was doing. All the same, with a bit of patience, you can get it to look ok, viz the above picture. When you are done, as with water gilding you can lightly brush the surface to get rid of any tags of gold and lightly burnish the finish, which is how I got the glow here. I'm not that pleased with it but as my first oil gilding experience, it's not unbearable. Stick to water gilding is my main advice...