How does a designer create unity?

Unity exists when your elements agree. Unity can be seen as the single most important goal of any design; to make your whole design more than the sum of its parts. Use the design principles of repetition, alignment, and proximity to add visual unity to your design and use contrast to add variety and interest.

To create unity or harmony in art, artists arrange similar components and elements of art to create consistency. Unity or harmony can also be created by merging the form and the meaning of the artwork together.

What is the Unity Principle of Design?