If there is something solid under it, rail, board, what have you, put your hat or any object large enough to lift the bellow horizontal. Some bellows are made with a Velcro strap going from the center of the front standard back to Velcro patches on folds further back toward the rear standard. On my RH Philips the strap is 1/2" wide by 3". When the bellow sags you can attach the Velcro strap where needed.
At my age I have found the operator often sags before the bellow.
John Powers