Only water a plant when it needs it.
To test for this, look at the soil. If it’s dark black, it is very wet and needs no further watering. If it’s lighter brown, press your fingertip into the soil or shift the topmost grains. If at 0.5cm deep it feels damp and looks dark, don’t water it.
How much water is required depends on:
Big, dry plants need 1 or 2 cups of water.
Small cuttings need only a splash.
Big palms may require a couple of cups every two days and shouldn’t be allowed to dry out fully.
If root bound, plants will take up water very slowly, so sit them in water for a few minutes. Heavily root bound plants will need 1/2 a cup of water every couple of days.
If a plant has completely dried out, it needs to soak up from water pooling underneath in order to saturate fully. If you’re unsure, pour a few drops of water on the soil. If absorbed immediately, it’s fairly wet and doesn’t need much, if any, water. If the drops pool or run over the top, it’s quite dry.
Try not to leave plants in water. If a plant already has water in its water dish, tip it out and do not water. For hanging plants, this will mean taking the bottoms off the pots to empty excess.