Most cyclers are familiar with the sweet sound of a gear clicking into place when they make a flick of their shifter. In a perfect world this isn't accompanied by any quick jerking and with little play. When those issues do start arising it means the gears are not indexed properly. Unlike that friend who spends every one of his Saturday nights indexing his VCR tapes, indexing the gears on a bike is a pretty simple process.
Prepare the Rear Derailleur
The first step that needs to be done is getting the rear derailleur in the right position to be set properly. Flip the bike upside down or put it on a stand so that you can shift into the middle or smallest slot. Then look for a small black oblong piece that turns, known as the barrel adjuster. Turn this barrel adjuster in fully clockwise and out a whole turn. The barrel adjuster effectively changes the length of the cable housing. There is also a cable mounting bolt near the back of the derailleur that needs to be unscrewed a turn or two to be released.
Check out our guide on setting derailleur limits to make sure the chain is set within proper limits.
Index the Gears
Pedal gently and push the derailleur until the chain moves to the second smallest cog. From here pull the shifter cable and tighten the screw so that the cable doesn't move. Pedal gently and shift to see if each gear falls into the cog easily. If the bike has problems upshifting, slowly release the tension in the cable until the transition is seemless. If the chain struggles a bit on the downshift, it could be due to grease and grime on the cables and they'll need to be cleaned or replaced.
Indexing the gears on the bicycle will make shifting that much more easier while at the same time ensuring all the gears available on your bike are accessible.