This is something my boss talked to me about because I work with customers almost every day and some of them have trouble hearing me most of the time. The sound should come from your diaphragm/upper chest and not just from your mouth/vocal cords. If you're by yourself you should talk to yourself just like you're talking to someone in a crowded room. If you talk and your upper chest barely moves or doesn't vibrate at all then you're more than likely doing it wrong.
Something my teachers did for me and the band group I was in before high school was they sat down on one side of the football field and each band member took turns yelling the chant we used (H! H! S! the initials for the high school) as loud as possible and they gave us a thumbs down or thumbs up from the other side of the field to let us know if they heard us or not.
Do something like that

Have a regular conversation with a friend on the opposite side of the room or talk with them with loud music in the background, be creative!
https://www.youtube.com/watch?v=hGWsH7zZWWg