Put your food in ziplock bags.
Close the zip except for a couple of centimeters at one end.
Sink the bag into water which will chase out remaining air.
Close the last part of the zip before you take the bag out of the water.
If you don’t have ziplock bags, use ordinary kitchen plastic bags and put bag clips on them after you have sunk them into water.
An alternative to sinking the bag in water, is to take a strawer and suck out the air with your mouth.