「英語の発音をするときは口の形をその発音をする形に整えなければなりません」
→ "When you pronounce something in English, you have to shape your mouth to match the shape of how one would pronounce it."
「整える」を"(to) shape"に訳しましたが、"(to) form (your mouth...)"も使えると思います:
"You have to form your mouth to match the way how that sound is pronounced."