The roof of your mouth is made up of a hard palate and a soft palate. If you touch it with your tongue, you can identify.   The hard palate stays in place. This is what's thought of as the roof of your mouth. It's the part of your mouth made of bone and covered with skin. It's encased between your teeth and attached to your skull.  Further back in your mouth is the softer, fleshier area of the soft palate. It can move up and down when you touch your tongue to it and it moves and stretches up whenever you speak, eat, yawn—basically anytime you use your mouth. Lifting your soft palate is key to controlling your voice, and can help you prevent singing through your nose. Imagine there is a ping-pong ball in the back of your mouth. You would need to have your soft palate lifted if there were an object back there taking up space.  Alternatively, you can do a half yawn. Notice that you lift or stretch your soft palate upward while you do this. Practicing this will familiarize you with the feeling of lifting your soft palate. You can also practice inhaling a soft K sound. This will naturally lift your soft palate, just slightly, so it is not as dramatic as the lift would be if you had a ping-pong ball in your mouth. Raise your soft palate and speak. You can try talking to yourself or doing something like reading a book out loud with your soft palate raised. It may feel and sound silly, but you will train yourself to be able to lift the soft palate on demand. This can also help you learn how to make noise with your mouth while raising your soft palate.

Summary:
Identify your soft palate. Practice lifting your soft palate. Practice talking with your soft palate raised.