Did you try over at r/pagan? Or r/paganacht
What area and time period are you looking for? 'Germanic' is pretty vague, and the religious customs probably varied a lot, even within smaller areas.
If you're interested in 1st century ad Germany (which you probably aren't, but I have the book so I might as well copy out a few bits), Tacitus records a number of religious practices in his 'Germania', though with what accuracy I don't know.
He mentions a couple of hymns or songs: 'In their ancient songs, their only form of recorded history, the Germans celebrate the earth-born god, Tuisto' and 'Hercules, among others, is said to have visited them, and they chant his praises before those of other heroes on their way into battle' (I've seen some authors suggest that Hercules is the Roman interpretation of Donar, both of them having clubs and being a kind of protector-god, more interested in humans than usual).
The religious dimension to punishment and warfare: 'Capital punishment, imprisonment and even flogging are allowed to none but the priests, and are not inflicted merely as punishments or on the leaders' orders, but in obedience to the god whom the believe to preside over battle. They also carry into the fray figures and emblems taken from their sacred groves.'
Their main gods and their sacrifices: chapter 9 again, the Roman names for deities are just how the Romans understood them, so Mercury is probably to be understood as Wotan, and Hercules, as I said, as Donar. I don't know about Isis, but Mars is probably Tiw (or however it was pronounced in that period).
Chapter 10 has various forms of divination, with religious elements. After this, which is supposed to be general information that applies to all the Germans (to what extant that's true I don't know) Tacitus then deals with the individual German 'tribes' or whatever you want to call them. There's a bit more about religious ritual, but the best known bit is probably his description of the Langobardi worship of Nerthus (mother earth) Chapter 40. Again it's mostly human sacrifice.
EDIT: the bits I copied are from the 1948 H. Mattingly translation, the bits I linked to are from the 1876 translation by Church and Brodribb (sorry, I know it's a lot harder to read)