Yes, boil the cup of water, dissolving the dextrose in the water. You want to make sure to cool it down before adding it to the beer.
Probably one of the best ways to mix in the dextrose-water is to first pour it into your sanitized bottling bucket. Then you will want to siphon your beer out of your fermenter into your bottling bucket, but make a coil at the bottom of the bottling bucket with your siphon hose.
This will allow your beer to mix and evenly distribute your added sugar into your beer while minimizing splashing. And Don't worry, Relax, and Have a Homebrew!