In most knives, the blade will be fattest along its spine, getting slowly thinner and thinner as you get the the cutting edge. Just before the very edge, a properly sharpened knife will feature a bevel that bring the blade to a fine point. If this bevel gets knocked out of alignment during normal use, you'll need to hone the blade using a steel.
A Honing Steel
A standard honing steel is made of a rod of steel that ridged along its length. The ridges gently guide a blade back into alignment as you draw the knife over it. A regular steel should be used every day before you use your knife.
A Diamond Steel
A diamond steel has a flatter length that is coated in diamond dust. Unlike a regular steel which will straighten a blade but not remove any material, a diamond steel will actually shave off a thin layer of your knife as you use it. It's not great for every day use, but it can extend the working life of your knife between sharpening sessions.
Step 1: Start at The Heel
Most first-timers find the vertical grip to be easiest. Hold the handle of the steel and plant the tip into your cutting board. Place the heel of the knife against the top of the steel at an approximate 15-20 degree angle.
Step 2: Finish at The Tip
Applying only light pressure, draw the knife down the steel, using the full length of the steel, and pulling across the full length of the knife, maintaining a constant angle.
Step 3: Begin Second Side
Hold the heel of the knife against the other side of the steel, again at a 15 - 20 degree angle.
Step 4: Finish Second Side
Drag the knife blade across the steel until the tip of the knife and the tip of the steel meet. Repeat with both sides until blade is honed—usually about 8 strokes per side.
Step 5: Advanced Method
For more advanced cooks, a floating hold like this one (shown here with a diamond steel) is faster, and doesn't require using up cutting board space. It is, however, more difficult to control.
Hold the steel out horizontally with your non-knife hand, and place the heel of the blade against the base of the steel.
Step 6: Advanced Method Continued
Pull the knife across the steel, maintaining a 15 - 20 degree angle until the tip of the knife meets the tip of the steel. Repeat on the second side.
Your knife should now be perfectly aligned.