If you already know German and Hebrew you are 90% of the way there. If you know Hebrew and another Germanic language (such as English or Dutch), you are about 50% of the way there. You can use textbooks and occasional translations or bilingual works to fill it the rest.
There don’t appear to be any good textbooks appropriate for frum Jews though Weinreich’s (which is basically pre-World War II) and others such as Zuker’s (multivolume with audio materials but recent) reflect the secular Yiddish as it existed before World War II. Frum Jews always spoke Yiddish differently (more use of Hebrew words, and without many of the “colorful” phrases that secular Yiddishists are proud of), plus World War II shifted the demographics since the Jews speaking the northeastern (Litvish, yeshivish) dialect had a much lower chance of survival than the southeastern (Galicianer, hasidiche) dialect. It might be nice if someone produced a textbook of the “living Yiddish” as spoken today in community’s where the children grow up speaking it as a first language.