When to use a sharpening steel:

Regular sharpening steel
A regular sharpening steel is used on a knife that is in good condition but could use just a bit extra sharpness. Actually it doesn’t remove any material from the edge but it realigns the microscopic metal parts at the edge. Therefore a regular sharpening steel can be used frequently without causing too much wear.

Diamond coated or ceramic sharpening steel
In contradiction to a regular sharpening steel, a diamond coated or ceramic sharpening steel does remove some material from the edge of the knife. Therefore it can be used on knives that cannot be resharpened using a regular sharpening steel anymore.
Diamond coated sharpening steels give the quickest result. Provided that they are used the right way, they give your knife back its sharp edge in just a few strokes.

How to use a sharpening steel
1 Find the right angle

The most important element in using a sharpening steel is keeping the right angle. As you use closely at the edge of your knife you will notice that the edge has a wider angle than the blade of your knife, generally somewhere between 35 and 40 degrees. (The smaller the edge, the sharpener the knife will be, but it also makes the knife more vulnerable. Global knives for instance have a cutting angle of about 20 degrees, which makes them razor sharp.) It is very important to maintain the right edge when you use a sharpening steel. If the cutting angle is 40 degrees, as with most German knives, you should keep an angle of 20 degrees between the edge and the sharpening steel. If the angle is too small, the use of a sharpening steel won’t have any effect and you might risk to make scratches on your blade. If the angle is too large, your knife will lose the it’s sharpness and will have to be resharpened professionally.

2 Keep the right angle
Now don’t let this discourage you to use a sharpening steel. Finding the right edge is use a sharpening steel more a matter of feeling than mathematic. To get this feeling, it’s best to practice first on an old knife, before you try to resharpen your 100$ chefs knife.

3 The strokes
The pictures above and below show direction of the strokes you must make. The tip down method as shown below is a safe way to use a sharpening steel, but personally I prefer holding it up because it gives me better control.
The best way to describe the motion is like ‘shaving off the surface of the honing steel itself’ (Wusthof). Only use slight pressure should be used.
Use 5 to 10 strokes each side, but never stroke each side more than once in succession.