I'm making up some cider and I thought I'd blawg it.
Here's a picture of what I have happening right now.
On the left is a bottle of water. On the right is a jar of apple juice, cider yeast and a tablespoon of honey. The jar is sealed closed and connected to the bottle of water with some 8mm aquarium tubing. The tubing is submerged in the water but not submerged in the apple juice. This makes an airlock; carbon dioxide from the yeasty apple juice can get out but oxygen can't get back in. Everything is sterilised before use. I got the juice by blending apples thoroughly and straining them with a piece of very fine mesh.
It's not a fancy setup. But it works.