Move the shifter/rear derailleur to the highest gear (smallest cog position), with any barrel adjusters (derailleur, shifter, downtube, etc) turned within a turn or so of being turned all the way in. Take hold of the cable with your hand and simply pull the cable tight, maybe do this a time or two to make sure everything seats in the right place. Then, with the cable in your hand pulled taut, making sure the cable is feeding under the anchor bolt in the right way, tighten the anchor bolt. Once that's done, spin the drivetrain and see if you get a shift when you move the shifter to the next-to-smallest cog. If it won't shift at all, add some tension by turning barrel adjuster counterclockwise.....keep experimenting until you get smooth shifts all the way up the cassette or freewheel, and back in the other direction. You're trying to balance cable tension against the derailleur's return spring to find the "window" of adjustment that allows responsive shifts in both directions.