No formal howto, just a number of us here that have done it.
Key aspect on the technical side is to find the most basic Compact Flash to IDE 44 pin adaptor you can. It should only support one CF card, and allow IDE master/slave to be set. This keeps it simple. Adapters that support multiple CF cards, or other types of media may work, but without seeing which adapter, it's hard to say how it would work.
The other simple aspect is the physical side. Find an adapter that mimics a 2.5 inch drive, including where mounting screw holes are. This eliminates the need to figure out a mounting solution. Though others may be as easy as some double sided tape.
If you find the right adapter, then the process mirrors the Drive Upgrade howto: http://www.riocar.org/modules.php?op=modload&name=Drive_Upgrade&file=index