We can describe the production of vowels in the following ways (keep in mind that these are relative descriptions and not definitions):
A vowel can be nasalized by lowering the velum to allow air to escape through the nasal cavity and the oral cavity at the same time. There are no GA vowels which are phonemically nasalized (as in the case with French, Portuguese, or Dutch). Rather, nasalization is determined by context.
2. Position of the lips
You can also change the quality of the vowel by rounding or spreading your lips. To pronounce /uː/ in GA you round your lips; to produce /iː/ you spread them. If you say the AN vowels /i/ and /y/ (of (Piet and Ruud) after each other, without putting a [ʔ] in between, you can feel your lips round and unround, while your tongue is virtually immobilized. Notice that vowels can vary in the degree of rounding: GA /uː/ is rounder than /ʊ/, while /iː/ is produced with the lips pulled toward the corners, like a smile.
3. Size of the oral cavity
We can also talk about the size of the oral cavity – or how open or closed it is. An open sound like GA /ɑː/ or /æ/ is made with the jaw low, and the mouth wide open. In GA, the jaw is generally opener than for AN open vowels like /aˑ, ɑ/. You may find it embarrassing to open your mouth sufficiently wide: the thing to do here, as always, is to watch and see how native speakers do it. A close sound like /iː/ is made with the mouth almost closed. The vowels /iː/ and /uː/ are both close and one way to distinguish between them is to say that for /iː/ the blade of the tongue is arched toward the front of the mouth and for /uː/ the back of the tongue is raised toward the velum.