This poss says it better
The first thing to do is to position your caravan, so that it is level side to side.
Probably the most important things that have to be level are the fridge and the sink, especially the *** as this will either not work or at best, not work correctly, if it isn't level. Use the ice tray in the freezer part of the fridge as this is a crucial part to get level, once that's level then find a work surface that matches it for level.
If the site owner doesn't care which way your caravan is pointing, then put a block under the uphill wheel and rotate the caravan by hand until it is level across the axle. Use a spirit level on a flat shelf or work surface in the van, if your not sure what is flat, use the base of the freezer part of the fridge.
If you have to pitch the van facing in a specific direction, then either place a pieces of old floorboard or a wedge under the 'downhill' wheel of the van or dig a trench under the uphill wheel (not recommended by me!) increasing each until the van is level.
Once you've got it flat across the axle (side for side) then you can unhitch it from the car (having firmly applied the hand-brake and level the caravan front to rear. Do this by raising or lowering the jockey wheel.
Once you are level in both directions, then you can lower the corner jacks.
NOTE: The corner jacks are not designed to carry the weight of the van. DO NOT ATTEMPT TO LIFT THE VAN ON THEM.