1st check your RD hanger alignment. If that is good continue. If not good, the drivetrain will never work well.

Set the derailleur in the middle cog (of seven that is easy, on a even number you have to chose one side or the other as there is no middle cog). Adjust the cable so that it runs the quietest on the middle cog. That should do it, unless there are spacing issues on the freewheel (been there with a Shimano 14-28 7 speed), or a kinked cable.