... I can never remember what I do as Germany... all I know is that it always works out.
The thing with germans is that their staple unit - the spear warband - is an extremely good and cost effective unit.
Build small strike forces and hide them in the forest near pivitol chokepoints or roads or cities - anywhere where you can land a good ambush.
By using ambushes you'll minimise you're casualty rates and therefore wont need to spend as much money on new troops.
In addition to this build farms, road, trade and ports in the order given to increase income.
Also throw all of the taxes to "very high" unless this results in a red face and rioting. If it's blue or yellow who cares? at least they don't riot.
Capture fertile lands that have access to the sea for sea trade AND better farming output - driving up your revenue.
You have 2 options in moving out from here.
You are either travel east and take any rebel cities in Russia while using small ambush forces to keep back the Gauls and Britons OR you push ALL your main forces on Gaul and push them back towards Spain.
Capture Samabriva... or whatever that Briton settlement is in Europe and drop the taxes to low so that you can quickly upgrade and get a port - enabling you access to the UK.
Break their line
Break their hearts
Break their legs