Adhesive size is a key part of leafing. it's what sticks the leafing to the surface. Paint on this milky glue, and wait for it to turn clear. (about 1-2 hours) Don't leave big puddles. Paint adhesive only where you want the leafing to stick.
Leafing is pretty easy. Its essentially 2 easy steps:
1) paint on the adhesive sizing, and allow it to completely dry to a clear, sticky surface.
2) Lay down your leaf sheet over the adhesive area, and brush it down with a dry soft cloth or dry paintbrush.
You can get a more classic look by applying a red base coat before applying the adhesive, and giving it an antique wash to highlight the details and give it an old & timeless look.