I like to describe unifying a piece of art as making a picture "match." You can do this by repeating a color scheme that you like, such as warm or cool colors. Sometimes artists like working in all circular or square shapes or by using a certain texture. You can also repeat by using all one art supply such as cut paper.
Another way to UNIFY a piece of art is by repeating words. We do a fun thing I call "word art" in grade 7!