1. Most textbooks (even the one from a frum source) tend to reflect Yiddish as it was spoken before the holocaust. The language has significant evolved. Assuming you want the modern language (spoken primarily among Hasidim in places such as Williamsburg, Antwerp, Bnei Brak) you have a problem. On the bright side, living Yiddish is heavily influence by English and Hebrew (whereas pre-war Yiddish was influence primarily by German and secondarily by Slavic).
2. For other languages a good way to learn is through newspaper and mass media (TV, movies). While there are Yiddish online shiurim, newspapers and some videos, they are relatively limited. Considering get Yiddish children’s books (the one’s sold to Yiddish-speaking children, not the translations into pre-war literary Yiddish). Live in a place where Yiddish is spoken by the children on the street.
3. If you want pre-war Yiddish (e.g. you aspire to be a historian), there are newspapers, recorded radio shows and lots books and textbooks. Arguably Yiddish has three “periods”, an early one in which it was basically German with Hebrew words thrown in, a relatively modern one in which there is a large secular literature, and the post-holocaust “frum” Yiddish which appears to be holding its own (based on the number of children growing up in Yiddish-speaking households).