Lift your tongue to the top of you mouth. Over time you will not need to do this, but when just starting out it helps get your mouth in the right position.
Push air past your cheek, forcing it out of one side of your mouth. Do this slowly at first. You will not hear a sound, you are just getting used to moving air through your mouth in this manner.
Increase the velocity of the airflow. You will hear the back of your cheek start to vibrate, creating a sound.
Manipulate your mouth and lips to create intelligible sounds from this noise. The movement of your mouth should be similar to the movements it would make in normal speech. Once you can lower your tongue and still make the vibrations, this will become much easier.
Practice making sounds until you can be understood. There is no fast way to do this, it just takes time and practice.