In fact, in English we close the glottis before all words that start with vowels. When you say the word apple, you actually start with a glottal stop. Thus, for English speakers no effort or practice is needed and you will naturally say a glottal stop before a vowel at the start of a word. The more important question would be, how do you say a word that begins with a vowel and NOT a glottal stop? But if I understand correctly, at the beginning of a sentence even words that start with a vowel get a glottal stop and within a sentence those words sort of blend in with the end of the previous word, so it's not so tough.
Since glottal stops are an unsound, you just have to practice closing that glottis before that vowel. It otherwise makes no sound.