How to learn German?
There's no doubt: learning German is a challenge. German is a highly complex language with a seemingly never-ending string of grammar rules that students need to learn and put into practice, and a rich but sometimes frustratingly context-specific vocabulary. However, the rules which are designed to make sense of the language are fairly consistent and logical, and German words allow for nuance and precision that gives the language its poetic depth. In short, the complexity of the German language is at once cause for frustration for those who attempt to learn it, and joy for those who master it.
The best answer to the question "how to learn German?" is thus: internalise and appreciate the logic of its grammar and understand the context-drivenness of its vocabulary. The rest is just practice ;).