Birds love to look their best and will need little encouragement to bathe, so allowing your bird to bathe is an important part of keeping it happy. All it really needs is daily access to a suitably sized container on the floor of the cage. Be sure to remove it after an hour or so to prevent accumulation of material from the cage floor. Another method of bathing is achieved by large dripping wet leaves (lettuce, spinach, kale) to the side of the cage next to a perch.
Most birds will enjoy rolling, climbing and playing with the leaves prior to eating them! You could also float the leaves in the water container once or twice a week to encourage the bird to jump in. Be aware, if you give your bird too much by way of green leaves, you may notice they have some loose droppings. Again, remove the leaves after a few hours to prevent them spoiling. As with anything new, it may take some time for your bird to get used to bathing. Be consistent about making a bath available daily and eventually your bird will realise it is a fun way to pass the time and get clean.
Don't be surprised if your bird takes a bath or shower then sits on its perch shivering. This is not unusual for birds and they will quickly return to normal. However, if your bird has accidentally found a large body of water (for example, a full sink) and has been immersed for a few minutes, it's important to dry and warm them as quickly as possible.