Depends on OS
It really depends on the OS you are using. Goggle both jailbreaking and activating iPhones. First your phone must be jailbroken (just use the method which works for your OS and skill level). Then the easiest method involves copying a modified lockdownd file to your phone (file and how to do this can be found in older posts and on multiple webpages) via which ever method you use to copy files to the system area of your iPhone (SSH, SFTP etc., some GUI's are available for these). Failing that several programs will also do this for you (iNdependancy for example). As I'm on Mac most of the stuff I use is Mac or UNIX based (command line stuff does not frighten me)