Simon, I have one for my Dorema awning. Fitting couldn't be easier(but its alot easier with red wine!!).You remove one of the end panels then the annexe has straps that loop over the end roof pole inside the awning and straps on the side that wrap around the legs (a spare leg is provided for extra support next to the van wall). The annexe is then put up and pegged down tent fashion. Had it up one year in bad weather and we didn't get any rain in the awning at all.
Thanks Dave