Make sure it's appropriate. Given it's sexual nature, freak dancing should be performed by two consenting adults in an atmosphere where it is acceptable. Don't go to grandma's Fourth of July party and start "hittin' it."
Feel the beat. Like any form of dance, feeling the beat and the rhythm is key. While most freak dancing is done to hip-hop music, it can also be performed to many other styles of music with many other tempos. Truthfully, it's up to the two partners.
Assume the position. There are two main freak dance positions. One entails the female rubbing her butt against the male groin area. (They are facing the same direction.) Then she can either slowly grind into his hips with her backside or shake her backside exceedingly fast depending on the beat of the music. The second position has the partners face each other. The female places her leg in between the male. They then "lock legs" and grind on each other, bringing their genitalia in close contact--again, moving fast or slow depending on the beat of the music.
Get a room. Freak dancing is an exceedingly provocative dance. As your dancing progresses (and you are two consenting adults), do not stay on the dance floor if things are progressing to the next step. While freak dancing is acceptable in clubs and other appropriate places, making out and fondling should be kept behind closed doors.