Start with clean hair. Make sure that your Yorkie's hair has been recently washed. Top knots will just get matted and ugly if you try to make them with dirty hair. Also, they will not puff as you want them to, but instead will stick together in small, sticky bunches.
Brush the brow hair thoroughly. There should be no tangles in the hair at all. It should appear shiny, glossy and completely smooth.
Pull all of the long facial hair back, starting at the corners of the eyes and moving upward. You will have a fairly thick skein of hair in your hands by the time you have gotten it all gathered together.
Work the mousse through the hair. Squirt a little in your palm and then use your fingers to ease it through the hair. Go light, as too much product will make the hair dull and heavy. Some groomers actually put mousse in the hair the night before or several hours ahead of time because they feel it makes the hair easier to work with. Others wait until they are ready to actually make the top knot to work the mousse into the hair.
Put the elastic in the hair as if you were creating a ponytail, but stop before you have pulled the "tail" all the way through the elastic band. The more you leave doubled up in the band, the bigger the pouf will be. However, it will also be harder to manage. This doubled up part will make a puffy knot of hair on top of your dog's head. The first few times you attempt this, you may have to try several times to get the hair to sit on top of the head. The mousse will help hold it in place, but you will also have to work with your dog's particular head shape to get the top knot positioned in a place where it will not flop over.
Spray the knot to hold it in place. Cover your Yorkie's eyes and also her nose, to prevent her from breathing the hairspray. Do not use too much.
Comb the rest of the pony tail into the fur on your dog's neck and back. Blend it into the long hair that falls down her sides.
Finish off with a hair clip or other decoration. Most people put in a small clip or hair bow. Your princess or prince is now ready for a night out on the town.