Hey,
Have a look at plugins/ssl/enomssl/PluginEnomssl.php. This should be open source and give you the starting point for a plugin.
Mainly, the doPurchase function is what is called when CE activates (purchases) an SSL certificate. You will also need to implement a doParseCSR function, which should return an array of: domain, email, city, state, country, organization, and organization unit, as well as a doGetCertStatus function to get the status of the purchase of the cert.