The vowels addressed here are the ones pronounced in the back of the mouth. They differ from one another, of course, based on how open or closed the mouth is when they are pronounced. You can start with the relevant post for some more information about the so-called back vowels, or give these exercises a try right away to see what you already know and can do.
Below are exercises which also cover the comprehension and pronunciation of back vowels. Watch this space for more to come!