Practice the 5 vowels repeatedly until you’ve got them down pat.
When you do it slowly, pronounce them and move your mouth as if someone 100 feet away was trying to read your lips; really enunciate and form them properly and slowly with your mouth. First we learn how to do it correctly, slowly, then we accelerate until we’re at full speed; that way we’re not only fast but also correct (native speakers have both, so must you!). A simple sentence (containing all 5 vowels plus the double “rr”) used to teach them to young children in Spanish-speaking countries is “El burro sabe más que tú” which translates to “The donkey knows more than you”. Check out the following short video for some help: