I was wondering what is the best way to build broader shoulders with dumbbells?

Dumbbells are great for building broader shoulders. Some great exercises for this would be the Dumbbell Military Press, and the Dumbbell Arnold Press.
Then, you could also do the Dumbbell Lateral Raise for isolating the medial or side portion of the Deltoid muscle.
Don't forget to work the back or posterior head of the Deltoids by also doing Dumbbell Rear Lateral Raises.
There is an exercise to specifically work the front of your shoulder appropriately named Dumbbell Front Raise but if you are doing pressing movement mentioned earlier you don't need to add in this exercise because they work the anterior or front part of your shoulder very well.

Just about all of the Fitrex programs will include these exercises with specific set and rep combination recommendations. Try them out!