I like to use a foam brush to apply the glue. When Michaels has a 10 for a dollar sale on these I stock up - and I don't care if I use a ten cent brush for one project. I start with a little bit of glue on the end of the brush.
Next, I apply the glue to the stamp. I prefer to use rubber stamps for this technique and I try as best as I can to keep the glue on the surface of the stamp and out of the crevices. You'll notice that the glue isn't perfectly "inked" on the stamp. And that's fine with me, I wasn't going for a crisp image. If so, I would have spread a layer of glue on a scrap piece of plastic and used it like a stamp pad.
Apply the glue, stamp onto your project and then go clean the stamp! It's important to clean your stamps right away if you're using anything other than ink. If I have a lot of glue left on the stamp, then I might stamp onto paper towel and wash with soap & water. If the glue has dried at all, I spritz it with the Best Cleaner Ever and wipe with my Perfect Cleaning Cloth. (I have an extra cloth dedicated to cleaning stamps!) Rinse with water.
For this stamp, I skipped the water step and went straight to the Best Cleaner Ever. (The cleaner actually works better if the stamp is not wet with water first.)
And since the Best Glue Ever dries tacky, you don't have to worry about your project. The glue will be ready when you are. Can you see the stamped area?
Finally, I apply the foil -- color side up -- and rub lightly over my stamped image. You can repeat this several times until the entire image is covered.