Retrieving process instance history.

I need to report on the steps that a process instance has been through in a winforms application.

This report is to be generated for a process instance that a user has access to.
I've looked at the K2ROM and cannot see a starting point into any historical data available in the K2ROM.

I do have access to the knowledge base, and have looked for information on this.

The filter service looks promising, but unfortunately I cannot find informaton on how to report on this.

I would appreciate pointers on where to start with filter service for this (assuming this is the correct API )

Thanks