Unless you are very adept and experienced, you and your bird would both be better off leaving beak trimming to her veterinarian. Your bird's beak is a very sensitive tool, essential to her survival, and used for such diverse tasks as eating, grooming, nest building, climbing, feeding the young, defense and communication. The beak is made up of horn-like tissue that grows continually, and therefore requires periodic maintenance.
Abnormal beak length interferes with eating, grooming and other activities, and in species such as parrots and budgies, an abnormally long beak can eventually curve down and penetrate the tissues of the neck.
In the wild, birds keep their beaks trimmed through natural abrasion from climbing, perching, and so forth. In their natural environment, birds have access to substances such as rocks to keep their bills worn down. In captivity, unless the owner provides a substitute, birds will have nothing to keep their bills worn down. We should keep in mind that as pet owners, one of our responsibilities is to provide a simulation of our pet's natural environment, which is why we should furnish bird cages with cuttlebones and mineral blocks to help with our bird's beak maintenance.
Aside from providing cuttlebones and mineral blocks, you can supply her with chew sticks, tree branches, nuts, pieces of wood and similar items. This can help drastically reduce the possibility of your bird's beak growing to abnormal lengths. It is unusual for a bird that has plenty of stimulation from such items to ever need her beak trimmed.
However, just in case your bird is unable to keep her beak sufficiently trimmed on her own, you will need to consider giving her a little help. Remember, though, that beak trimming is definitely something that should not be handled by an inexperienced owner. Beaks can split or crack, and if they are cut too short, they can bleed extensively. To make a comparison, a bird's beak is as sensitive and fragile as your teeth. You certainly would prefer to have an expert dentist take care of your teeth! Your bird's veterinarian should handle beak trimming during routine visits and checkups.