[DavidG]

The bending, with or without steam (let the cardboard dry thoroughly before installing if using steam), will go a lot easier if you do not fight the tensile strength of the upholstery fabric. Just peel the fabric off the back on the edge that you're bending and reattach it with contact cement (lightly applied) once you've bent the cardboard to where you want it. Do not be intimidated by peeling back the fabric and re-gluing it as that how it is attached in the first place.