You'll need:
* A harmonic balancer holding tool: This specialized tool fits into the harmonic balancer's teeth and prevents it from rotating while you tighten the bolt. Without this, you *cannot* safely tighten the bolt.
* The correct size socket: This will depend on your engine (2.0L or 2.4L), so check your repair manual or a parts store to confirm.
* A breaker bar or ratchet with sufficient leverage: The harmonic balancer bolt is very tight.
Procedure (in short):
1. Get the right tools. This is the most important step.
2. Position the holding tool securely. Ensure it's firmly gripped onto the balancer.
3. Use the socket and breaker bar/ratchet to tighten the bolt clockwise. Tighten to the manufacturer's specified torque. This torque specification is *extremely* important and can be found in your repair manual (or online through a repair manual site like Chilton or Haynes). Using too much torque can damage the crankshaft.
Warning: Improperly tightening this bolt can cause severe engine damage. If you are not comfortable performing this repair, take your vehicle to a qualified mechanic.