Put all the ingredients in an air-tight container.
Shake the container well until frothy on the surface like in the top photo (tumblers with a lid like the one in the photo or an air-tight plastic container is good).
If you want to make banana lassi, mash a ripened banana roughly, add it to the mix and shake.
Story Behind this Recipe
In India, people put the ingredients in a clay pot and pound it rhythmically with a stick to blend. I don't have a clay pot, so I shake it in a tumbler covered with a lid (you can use air-tight containers as well.) You can make this whenever you want to drink it, and you don't need to wash cookware afterwards, so this is very convenient.
The trick is to shake it really well to make it foamy. The end result is different compared to if you just stirred the ingredients. Finely crushed ice works best. Only use a small amount of ice, enough to cool the drink. I used granulated sugar, but other types are fine too.