Use a hyphen between the tens and units number when writing out the numbers twenty-one to ninety-nine in words. Just like that!
Do not use hyphens for other numbers.
Correct: Two hundred fifty-six
(Hyphen between tens and units only)
Use a hyphen between the numerator and denominator when a fraction is written out in words and the fraction is an adjective.
Incorrect: Two-thirds of the Senate overrode the veto.
(Here two thirds is a noun, not an adjective.)
Correct: Two thirds of the Senate overrode the veto.
Incorrect: A two thirds majority overrode the veto.
(Here two thirds is an adjective modifying majority.)
Correct: A two-thirds majority overrode the veto.