Well first you need a kernel the will overclock. CTMOd has one already and CM7 has one that can be added.
Once you have the proper kernel, you just open up Voltage control and where it says Max, try turning it up slowly. Most devices will let you overclock to 900MHz while some will even go all the way up tp 946MHz. You can leave the Min set at 300MHZ or try turning it down to 220MHz.
You can also adjust the I/O. I usually set mine to SIO while others use NOOP or DEADLINE. Test them to see which works best for your phone. There are tutorials available in the link that will explain what each setting does.
Check the links in this post:
http://androidforums.com/4224669-post65.html
After you have set them where you want, make sure you tap the settings and save your changes. You can choose to set them at boot or init. I use int because it sets them when the phone boots up. The set at boot setting will apply the settings after the phone has booted. The init setting is faster.
IF YOU START EXPERIENCING LAG, REBOOTS OR FREEZING, TRY TURNING THE MAX SETTING DOWN. THE PHONE MAY NOT CAN HANDLE THE HIGHER SETTINGS.