Once danger of frost has past, plant them out. You should have a prepared bed for them to grow. Make a shallow furrow, plant them.

If by indoors you mean in the house take care to make sure they have plenty of light until you plant them out or they will grow tall weak and spindly. Don't put them straight outside from a warm house, get them used to cooler temps first, perhaps on a sill in an unheated room. Or put them outside during the day and bring them in at night for a couple of weeks to harden them off.

If they are in a cold greenhouse they can go out when frosts are over as B says..