To get started, you need two bronzers -- one that's darker than your skin and another one that's shimmery and light -- plus a soft powder brush. You should put your bra on first and your dress or shirt if possible. You can just cover the neckline with a paper towel or tissue to keep from getting makeup on it.
For the first step, dip your powder brush in the dark bronzer and shake off the excess. Start where your bra strap meets up with the cup and apply the bronzer along the curve of that area and then down the inner edge of your cleavage. Do this on both sides and the crescent shape will make your breasts look more voluptuous and round.
Next, brush the light bronzer down the middle of your cleavage. This shade catches the light and creates depth. Sweep some across your collarbones too, which draws the eye up so your boobs seem to sit higher. Then, wipe the powder brush clean and go over the areas with a gentle circle motion to blend any stripes. You can also dust the upper half of your breasts with the lighter bronzer to offset the darker color in your cleavage. Just be sure you don't use too much and that the powder isn't super shimmery.