If you want to get a deeper voice instantly, two things you can look into are your posture and breathing. Poor posture will without a doubt impede on the quality and sound of your voice. Making sure you are not hunched when you talk (you have to make a conscious effort to do this – many of us hunch our backs out of habit, so you have to be pay extra attention to your posture when you’re talking) will open up your diaphragm and allow air to move more freely, and in turn you will instantly be able to speak with a clearer, deeper voice.
Another thing you can do is check your posture in front of the mirror to see if it’s okay and if any improvement is needed. Talk to yourself in the mirror and watch yourself speak whilst maintaining different kinds (from good to bad) and whilst breathing from different parts of your body. We tend to breathe through our chest, but in order to speak more deeply and more clearly, we need to breathe even deeper. We want to be breathing from the diaphragm, which is around about where our belly is. You know you’re doing it right if your chest and shoulders don’t move when you breathe. Breathing from your diaphragm will not only help you to speak in a deeper voice, it will also make it easier for you to maintain a good, straight posture.