Donald Duck's voice, as performed by Clarance "Ducky" Nash in Walt Disney movies and shorts for 50 years, is created by a process called buccal speech. Buccal speech is the process of forcing air into the space between the cheek and the jaw, causing the arches in the back of the mouth to vibrate and create sound. Over time, you can learn to control the rate of air flow and manipulate the vibrations in order to mimic speech.
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.
The back of your cheeks should be vibrating, not your throat. If you feel pain in your throat you are forming the sounds incorrectly and could injure yourself.