There is no better teacher than experience. Falling short of that, read the source code. This page contains links to download the full version of the Java and Squeak code examples in Streamlined Object Modeling.
The above link is to a standard ZIP file containing Java source. To use the source simply extract the files into a directory of your choice. (The archive file contains path information to create the correct package/directory structure.) Since Java does not come with its own integrated development environment, you can either use a simple text editor to view the individual files or use the IDE that suits you. If you do not have an IDE of choice, a quick web search will turn up several. Here are some links to get you started:
The link above is to a standard ZIP file containing a single .st file. To get this source to be readable in your installed Squeak environment, follow these steps: