Water them when they get dry. It's really as easy as that. Small pots dry out much faster than larger pots, obviously, so with really small pots you may have to water twice per day or even more. I had some seedlings in 2 inch wood fiber pots and they were drying out every few hours, which got to be a pain. So I repotted them into 4 inch plastic pots and now they are holding water much longer.