How can I stabilize the footing of a ladder?
The question doesn't have a specific answer since ladders, ground conditions, and tools-on-hand vary. Here are some ideas.

Use integrated spikes. Many extension ladders have feet which can be spun around to expose metal claw edges. These are not ideal on pavement, but work well on softer surfaces.
Dig small holes for the ends of the legs.
Park a vehicle bumper against the ladder, or against a strong object which is against the ladder and can't flip over or slide out.
Anchor the legs to a fixed object with the tension oriented under the ladder.