I suggest you use the Osmocote with the shaker top. Then just shake the product on top of the soil around the roots of each plant. The granules should  be sparse enough so that they do not touch. One capful of product does a 2 foot by 2 foot square so from your image it looks like one capful would do 4 feet of bed as your bed looks about a foot wide. Once you sprinkle the Osmocote onto the soil go back with a light rake or hand claw and work the granules into the top layer of soil. Then water in well and you are done. The Osmocote will feed your annuals all summer long. Looks like you are going to enjoy a very colorful display. Keep growing, Marianne Binetti