Firstly, you need to decide (and make clear) if you are in supoort of the statement: was Betty Friedan really the most important, or will you be arguing that another individual/ group were more influential? Then, you need to outline your essay structure.
Friedan was important in the 1960s and 70s, popularising second wave feminism and gainign support for the post suffrage movement. 1963 The Feminine Mystique. You need to demonstate her importance persuasively (particualrly if you are arguing that she is the most important.) Then provide a counter argument- what were her limitations?
Gloria Steinem. As above. Explore her importance, and counter argument. Remember to stay with your line or arguement throughout.
Elizabeth Cady and Susan B Anthony in the 1860s and 1920s: The importance of the suffragette movement in paving the way
Fianlly, for that extra sparkle (an A*) it's good to include some information that shows you have gone beyond what the average student can say. In your introduction and conclusion, bring in an extra fact. For instance, you can talk about how Friedan's ideas about seperate spheres was a central issue, as demonstarted by the controversy brought by Hilary Clinton when, in 1992, she patronised housewives by claiming that she could have 'stayed home and baked cookies' but had instead chosen a career.