After the first stage main engine cutoff, cold gas N2 thrusters are used to rotate the booster into the direction of flight, they reignite 3 of the Merlin 1D engines. (With 9 in the Octaweb alignment, the center and two on either side of it allow a 'line' of engines to fire).
They use this to kill forward momentum. They continue arcing upwards (since vertical and horizontal momentum components are independent) until they start falling down again.
Finally they use the hypersonic grid fins at the top of the upper stage, the cold gas N2 thrusters in combination to try and steer the booster to its target. Time will tell if they have sufficient control to hit a small target that's 300 feet long by 170 feet wide (91 by 52 meters) precisely enough for Falcon 9's leg span of about 70 feet (21 meters).
Once they approach their landing surface, in the last few seconds (10-30 seconds?) they re-ignite the central Merlin 1D to decelerate from terminal velocity to a landing. Hopefully down to 4.5 mph (7.2 km/h) at the time of touchdown, a number that was mentioned during the recent CRS-5 pre-launch news conference. If not, bad things happen.
Source: space.stackexchange.com
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