Keep them watered like you would any seedling. When they get a couple of inches tall you can gently dig them up and transplant to pots or another spot in the garden. For easy digging use a long handled iced tea spoon, scoop one up without disturbing the one next to it.
I had petunias growing in the ground from plants in a window box I had two years earlier. I've also had some pansies come back in the ground from hanging baskets the year before. I just watered them and took care of them like any other flower and they flourished (I didn't move them - they were in a perfect location, actually) I figured if they can stand a New England winter or two, they were worth keeping alive!
