Stand or sit with your back straight and with your arms at your side. This position allows the air to pass though up through your lungs and out of your mouth with greater ease.
Begin vocal exercises by breathing slowly inward and singing scales as usual, but using the vowel sound "ah" to go from the bottom of your range to the top. Be sure not to strain your voice at the top of your range, or you might damage your voice instead of strengthening it.
Grab a phonics dictionary if you have any trouble remembering your grade school grammar lessons, as it would defeat the purpose of the warmup to mispronounce each vowel sound.
Sing a set of scales using short vowels sounds. For example, sing a short "a" as "ah" and continue with the sound to the top of your range. Follow with a short "e," "i," "o," and then finish with "u".
Continue the voice training warmup exercise by using the long versions of vowel sounds. Begin again alphabetically singing your scales with the long "a" sound of "ay." Next, simply move on to each long vowel sound alphabetically.
Change up the exercise by practicing in different languages. Learn the vowel sounds of a few other languages and place them into your scales one by one until you've mastered each sound. This is especially helpful as a warmup when working with more difficult vocal pieces.