Giotto's technique

I was especially interested in how the gold leaf was applied. What truly intrigues me about Giotto, Simone Martini, Fra Angelico, Botticelli and most other early Renaissance painters is just how stylized... one might even say "abstract"... their paintings were as opposed to the illusionism of Baroque and post-Baroque painting (excepting the Rococo, William Blake, Ingres, etc...) prior to Impressionism.
Awesome to think of the skill, time and learning involved, very humbling. Thanks for posting this.
The technique is called water gilding, with a little mordant gilding thrown in. Much more complicated than oil gilding. It's fully described in Daniel V. Thompson's The Practice of Tempera Painting, as well as his Materials and Techniques of Medieval Painting, my favorite art book.
No matter the type of gilding, leaf is best applied with a gilder's tip. It's possible to apply thicker imitation leaf by other means (though I wouldn't), but gold leaf is only three millionths of an inch thick and is bound to give trouble otherwise. It is also for the best to use a gilder's cushion and gilder's knife to cut the leaf to size if needed, otherwise much expensive leaf will be wasted. The process is very old. Best to go with what has worked well for centuries.

Water gilding, in which the leaf is burnished with an agate burnisher (formerly a dog's tooth) gives much more an appearance of solid gold than oil gilding, which is really just the application of leaf to a varnish with driers (gold size) which is very slightly tacky.