This process of polishing, sanding, and putting on the gesso is repeated many times. Now the artist is ready to make a cartoon of what he wants to paint on the panel for the finial picture. A cartoon is a drawing of the subject the artist wants to portray. The cartoon drawing is the actual size of the final painting. I think you could say that for egg tempera, having a nice gold leaf background was very trendy.
Gold leaf is extremely thin and very hard to handle.
To apply the gold leaf, you first put a thin layer of the egg white mixture on the panel, then pick up the piece of gold leaf and put it over the coated panel area. Next you take your brush and gently smooth the gold. This is very tricky and takes a while accomplish. The gold leaf is made out of gold or metal that is pounded into very thin sheets. Some artists stamp different shapes in to the leaf.