"I do not like your girlfriend."
Translation:Ich mag deine Freundin nicht.
This tripped me up as well but I'm gonna keep things pretty simple - if there's an object, the "nicht" goes after it. No object, then "nicht" comes after the verb. I'll probably be wrong 10% of the time using that rule but hey! I'll be right 90% which is more than most Austrians :p
Im pretty sure you would only use that if you are saying something else that you do like. "Ich mag nicht deine freundin, sondern seine freundin."
But if you just want to say i dont like your gf, you would put nicht at the end to negate the whole thing. In general if you aren't using sein (to be) then you will put the nicht after the noun in a possessive case. If theres no specific object, you'd use kein.
One uses the feminine form deine when one is modifying a feminine noun. Similarly deinen and dein refer to masculine or neuter case nouns. Deinen furthermore refers to the accusative (direct objective) declension of a masculine noun. To make matters slightly more confusing, deine is also the plural form so that it is used with all plural nouns. A few examples: Ich mag dein Haus (I like your house}. Ich mag deine Freunde (I like your friends). Ich mag deine Freundin (I like your girlfriend). Ich mag deinen Freund (I like your friend). I hope that helps.