I love compound words (two words functioning together as a unit). They come in three flavors:
–words joined together without a space: homework, overdone
–words joined with a hyphen: red-headed girl, green-eyed boy
–words joined WITH a space between them: overly styled hair (overly modifies styled, they both together describe hair)
There is a certain amount of leeway allowed in the way you write certain compound words, mainly based on the style guide and dictionary used (bestseller vs. best-seller, anyone?).
But there is one hard and fast (or hard-and-fast) rule: never hyphenate compound words in which the first unit is an adverb. Most end in -ly, like overly, beautifully, or unfortunately.