Using an individual pot, fill it with compost, a few seeds and moisten. Stretch a polythene bag around the pot. Some small bags fit gardening pots well. It needs to create a tight seal, so if the bag is larger than the container, use a rubber band or tie to seal it.
Ideally you do not want to bag to rest on the small seedlings, to prevent this you can use a small pea stick to prop up the bag. This will work well for many seeds.