During my times teaching students, whether they are just starting out or have been practicing English accent for decades, it's always important as a teacher to go back and visit the essential vowel phonemes before continuing to more complex consonants sounds such as the American R or "Th" sounds.
Why you might ask? It's because 1.) Vowels teach us about the essential mouth positions in English such as the Schwa that make learning consonants easier and more intuitive. This last part is most important, because far too often advanced students who can comprehend and articulate complex ideas easily, still make simple errors in their pronunciation. These errors can cause you to be misunderstood.
So then what are vowel phonemes and why are they important? A Phoneme is any of the perceptually distinct units of sound in a language that distinguish one word from another, for example p, b, d, and t in the English words pad, pat, bad, and bat. In the case of what we will be covering, you can think of Phonemes as being short or long. Short mean they have one mouth position, one tongue position, one jaw position and therefore is a static sound. Long means they have more than one mouth position, more than one tongue position, and is a moving sound.
A good example is comparing the Long O vs the Short O. Take for example, blow vs block. Say it a couple of times and put your hand near you Adams Apple. As you say blow, the you can feel the Adams apple in your throat moving down, and your mouth ends in a "w" position. In block, the Adams apple hardly moves and the mouth stays much more static compared to "blow".
The more you practice the more you will be able to hear the difference between these long and short vowels. More importantly, you will be more aware of the different positions in your throat, tongue, and mouth as you speak! This will make progressing to more complex vowels and consonants far more easier.