Female to Male Voice Training
While it is generally accepted that testosterone treatments will deepen a trans man's voice and voice training isn't necessary, this isn't strictly true. There are many reasons why voice training can be of use. Not all men will opt to go through hormone replacement therapy or are unable to pay for their hormones, and so must find alternate means of passing. At around the age of twenty-five, the cartilage that houses the larynx will begin to harden, reducing flexibility. Transitioning after this point will make changes in the voice more difficult, but not impossible. The vocal changes that take place during a natural male puberty happen when the body is young and not fully developed, as well as with a gradual increase in testosterone. In contrast, transitioning FtM individuals are usually started on relatively high doses of testosterone that are then gradually lowered.
Some trans men will find their voices deepening to their satisfaction with no effort or thought after they start HRT. However, because this isn't the case for every man, it's a good idea to consider some voice training and precautions.
As any professional singer could tell you, abrupt and drastic changes to how you use your voice can damage it, often permanently. Gradual change is best when it comes to the voice, which is one reason why beginning HRT with lower doses of testosterone can be of great benefit. The drawback to this is that a slower hormonal transition makes passing more difficult. If you haven't started HRT yet, it would be wise to research it carefully and talk to your doctor before beginning.
Using the Diaphragm
Speaking with your "chest voice" rather than your "head voice" will automatically drop your voice into a slightly deeper register, whether you're taking hormones or not. Learning how to breathe with the diaphragm consciously will also aid you in projecting your voice, which gives a more powerful tone and helps with traditionally masculine speaking.
If you are unfamiliar with diaphragm breathing, the best way to start is by laying on your back with one hand on your chest and one on your stomach just below your ribs. As you breathe in slowly through your nose, the hand on your stomach should rise while the hand on your chest should remain largely stationary. As you exhale through your mouth, the hand on your stomach will descend. If you wear a binder, the compression of your ribcage may have restricted your lung capacity, making this exercise both difficult but also even more beneficial.
Practice diaphragm breathing while laying down for no more than five minutes a day until you are comfortable with it. Then graduate to doing it standing up against a wall. Eventually, you will want to practice it without any support and only a mirror for a guide on your movements.
Once you feel comfortable with diaphragm breathing, you can begin practising humming as you exhale each deep breath. Don't alter your breathing beyond making sounds as you exhale. Your voice will likely sound uneven and even crack a bit when you begin doing this, but eventually you'll find the control to achieve a richer, deeper tone. At that point, you can begin practising speaking from your chest.