The majority of Selective Schools in Essex are united under the banner of the CSSE: the Consortium of Selective Schools in Essex, or simply, the Consortium. There are a few schools outside of this system, however: most notably the King John School, Benfleet and the Chelmsford County High School for Girls, the relevant application procedure for each can be found on their respective websites.
For Consortium Schools:
All this is true as of the time of posting. Check here for updates.