Place the barrel top rib down on a cushioned surface. Use a hardwood wedge (half of a wooden two-piece spring-type clothes pin works well) and firmly push it between the tip of the forend and the bottom rib of the barrel.
Hold the roll latch release forward while inserting the wedge.
The last time I needed to do this it took 2 people. You'll need to brace the breech of the barrel somehow.