Question

A writer should hyphenate a compound modifier that

Answer 1

Comes before the noun it modifies.

Answer 2

A writer should hyphenate a compound modifier that comes before the noun it modifies. 
Take a look at this example:
No one likes a two-faced person.
As you can see, the modifier two-faced is hyphenated, first of all because it is compound (meaning that it has more than one word), and second of all because it comes before the noun person which it modifies.