It's quite simple.  The negative effects of a good crack on a cymbal can be almost eliminated, the sound quality will greatly improve, and the life of the cymbal will be extended with this simple process.  If you can purchase a cracked cymbal with this process already completed then you'll have a great and inexpensive cymbal without the hassle.  If not, get your hands on a good cracked simple for cheap then bring tremendous life out of an oldy but a goody with the following steps:

Here's what you'll need: 

1- Power drill.  (If it must be a chordless the more powerful the better.)

2 - Metal/Steel drill bit

3 - Large towel

4 - #2 pencil

5 - Good eye sight

6 - Patience

What to do:

1 - Place the large towel under the cymbal. This will collect the bronze debris which if stepped on can act like glass particles. (You can imagine what happens if you step on this stuff.)

2 - Locate the crack on the bottom of the cymbal and circle the ends with a pencil.  To ensure you have the very end of the crack bend the cymbal to locate.  Sometimes the crack can be a thin hairline crack that is barely visible.  Bending the cymbal just enough to find the end of the crack will make it easier to locate.

3 - Start to drill at the very end of the crack to stop the crack. Ensure that the drill bit used is at least 4mm larger then the diameter of the crack. Drill all the way through pushing in and out several times to ensure a good clean cut is made through the cymbal.

* Note : Make sure that you drill the bottom side of the cymbal.  Othewise you may bend the cymbal out of shape.

Your result will be a great sounding cracked cymbal that will last you much longer for a few bucks and a little effort.