I have never cleaned one of the EFM hot air units, so I don't have any specific information about it. I am also not aware of any manuals or guides that would help you with it. EFM has dealers that are trained to service these units, and they assume most service will be done by the dealer, rather than the homeowner - hence the lack of manuals and videos.
Generally speaking you should be able to do a decent job with a couple flexible handle brushes, a vacuum, and maybe a bbq brush. Shut the unit down, let it cool off, clean out the pot (great time to clean the air holes in the grates), scrape/brush out any fly ash from the fire side of the heat exchanger, remove the flue pipe, clean the flue pipe, and clean the ash chamber at the back of the unit (where the flue pipe connects). Basically the entire path that the flue gasses travel needs to be checked and cleaned. Since this is a hot air unit, I assume you would want to make sure the hot air side is not plugged with dust. This would not cause CO or fumes to get into the house, but it could impact efficiency.