Actually there is an example file inside ASM folder that you download. It's called Helloworld and it's located in examples subfolder. It shows how to compile (generate from scratch) .class file that corresponds to simple hello world app. It also shows how to get date from .class files but it's another story.
Maybe this is'n the best way, but when you need to start with java byte code generation and you need some basic examples it's a good idea to have a look at ASM and the examples that are bundled within standard package.
Moreover Groovy uses ASM to generate its code :)
