Not sure how this’ll be helpful, other than adding useless facts. You asked for DIY. Anyway here it is..
Years ago I’d taken a few singing lessons—more like local karaoke classes but the teacher was an actor. I was told that vocal articulation is a habit you acquired in childhood. You copy everything from people around you, and this includes the way you vocalize or express yourself or get your point across. If your house is typically quiet and your parents tend to whisper, or they somehow don’t value your voicing out opinion, you tend to become soft-spoken as well. Like you swallow your voice. Or if you’re a thinker type, or you’re not so grounded in your body, you’d tend to speak from inside your head, as a metaphor.
Another thing is that vocalizing is.. psychological? It reflects your state of mind. When I speak English, my 2nd language, I’m not nearly as natural or fully voiced as when I speak my native language. There’s always that little mental twinge, like an inferiority mental block, that holds me back. I could never express myself in English spontaneously as if it’s coming from me as a whole… hard to explain.. As if, if you’re inhibited in some way, or if you don’t completely believe what you’re saying, you’re not going to sound resonant. An example my teacher gave me: some people can’t say “no” because they come from a family that don’t value their opinions, so they don’t have a “voice.” People are more silent in some cultures than others, especially in high-conformity cultures. I’m not a voice expert so could very well be wrong here. Please correct or refute my statements otherwise..
So in singing lessons what the teacher often did was to have me mimic his vocalizing. He would sing along so that I could “tune in” to him. But my habit was so well-formed that once I left the class I could hardly replicate the resonant, non-flat vocalizing that we practiced in class. It takes a lot of practice and whole-body awareness, and to this day I could vaguely remember how to form that space in my mouth for the purpose of singing. I didn’t grow up voice-conscious, so all this wasn’t easy for me.
I really think you should find a professional voice coach, maybe take 2–3 classes just to get a starter feel, because how else are you going to correct yourself? Vocal articulation, let alone teaching it, is an expertise. You could check out Fitzmaurice Voicework in case you’re interested in workshops. Or ask lifeflame or your theater friends.. before they start charging you for it..