Apply the ground cover while the surface is not angled or vertical. Or, if it can't be helped (I make hilly terrain out of 'ground goop', a mixture of Portland Cement, Plaster of Paris, fine-ground vermiculite, and at least two masonry dye powder pinches added before adding water. I mix about two pints at a time and slather it over hot-glued window screen material.), spritz glue mix over the surface, and blow...literally purse your lips and gently blow...ground foam off of a pie plate or a slightly creased, V-shaped, bit of cardboard.
When I don't want to do this, even if just to switch it up a bit, I'll make a removable module frame of scenery, often a mountain under which is part of staging or something, and when I go to add the greenery, I take it outside, prop it up so that the angled or vertical surfaces are mostly level, and drop ground foam onto the pre-glued surface. This can't be done in all conditions, as you would have guessed, but on some days it is doable. Otherwise, on the floor of the train room and have your shop vac handy.