FreeForm and extension hooks

Hi. I’m trying to add an extension hook in EE2 to SolSpace’s FreeForm Module so it submits information to Campaign Monitor. I’ve found what the hook is at http://www.solspace.com/docs/detail/freeform_freeform_module_insert_end/ but I’m not sure how to go about actually writing the hook.

I’ve looked up the documentation on writing extensions at http://ellislab.com/expressionengine/user-guide/development/extensions.html but I couldn’t find how to apply it to writing an Extension for FreeForm.

Someone else has already made an hook here http://www.solspace.com/forums/viewthread/4676/#26825 but I presume it must be for EE1 as I couldn’t get it to work.

Any pointers on where to start with this?