Lie on the floor or stand against the wall and press your hands lightly against your stomach, beneath your ribcage. Breathe in and out, so that your hands rise and fall. This is how it feels to breathe from your diaphragm; which fills your lungs more than breathing from your chest. You will need to push your air from that part of your abdomen while singing, to provide the support needed for a large sound.
Perform lip trills. Allow your lips to close gently, without pressing them together. Breathe through your lips, allowing them to vibrate with the force of the air. You will find that if you are using support from the diaphragm, this is a much easier exercise.
Sing scales on a nasal "nay" sound. Make it as nasal and whiny as possible, as the force of pushing sound through your nose will force your chords to close more tightly.
Buzz on a closed "ng" sound as you perform a scale. The closure again forces the chords to close more tightly, in order to produce the sound.