Boil the sugar in a small amount of water. You need it to dissolve properly to be evenly distributed, plus the boil will help insure that your beer doesn't get infected with any unwanted microbes.
I think it is more common to mix the sugar into a couple of cups of boiling water to create a syrup solution. This will dissolve/mix with the beer more easily.
I nuke 2C of water & add the right amount of dextrose & stir till it clears. Cool it down to 70F or so,then add to priming bucket. It's thin enough to mix easily.