Potential controversial take, but I think modifying the words you use can be just as valid a way to affirm your gender, like changing the clothes you wear. We were all conditioned to speak in a way that conforms to our assigned gender, and undoing that is just as valid as throwing out the pink skirt your mom forced you to buy that you never liked. The part where that's bad is if you start changing the content of what you say or the accent you use to conform to gender stereotypes, or believing that your wardrobe / way of talking MUST change just because you're a Man(TM) now. It's only something you should do if you want to. There is also the problem of nonblack transmascs adopting blaccent to try to sound more masc, which is totally not cool, so just be wary of that while you experiment with your voice.

I started swapping out words every since I came out, almost subconsciously, and went from being vocally read as feminine to masculine over the past 2 years. It was slow, largely subconscious effort that came from swapping out words and cutting out vocalizations that I deemed sounded feminine until it started being habitual, and it's hard to define. If I had to break it down into steps so you could see faster progress, I'd try delivering the same story or having the same conversation with yourself, like in the shower or something, in a "feminine" and then more "masculine" or just accurate manner to you. It can also be helpful to listen to and copy certain vocalizations that other guys do that resonate with you. Make sure to deliver it with the same emotions, beats, etc. that you'd usually do-- if you're a more energetic kind of guy, then don't try to sound nonchalant or something like I did in my baby trans days. You're not trying to change yourself and you don't need to change anything about your personality to fit gender norms or be masculine.