The best way to water bonsai is from the top. It sounds obviuos but it is surprising how many people submerge the pot underwater everyday. Having said that, however, if you have an imported bonsai that is planted in a clay like soil, then standing the bonssai in a bowl of water once a week or thereabouts is beneficial to making sure it gets a thorough wetting. (They should really be repotted).
More trees are killed by incorrect watering
You can use a specialized Watering Can or any small watering can with a long neck and a fine rose. water thoroughly until the water comes through the hole in the pot, but how do you know when to do that?
Expose the top of the soil with your finger or a stick.
You could also push a small stick into the pot all the way to the bottom and leave it there, that way you can take the stick out everyday and see how wet or damp it is at all levels in the pot.
Not enough water can kill a bonsai in a few days, whereas overwatering takes longer.
A dry root becomes brittle and can’t absorb water, whereas a waterlogged root becomes rotten and spreads back into the healthier root thereby depriving the tree of more healthy root to take up moisture.
So your Bonsai leaves are losing colour and wilting! Is it underwatering or overwatering?
Most beginners think it is underwatering but now we know it could be either.
So if your bonsai doesn’t respond to watering after a day or two then it must be due to overwatering.
How to dry waterlogged roots
Carefully lift the tree out of its pot and plant it in a larger container of dry sharp sand, keeping the rootball intact.
Use a stick to check if it is drying out, and only water sparingly at first when it has dried out, you may need to trim the ends of the roots if they are rotting badly. This metod may or may not work but it is worth a try.