Building
First off, you need to decide whether you want to just build the plugin, or actively contribute to the plugin.
If you just want to build it, you can obtain the source-code directly from the rlf/uSkyBlock repository, if you want to contribute, you need to fork the repository, and fetch the source from your own (newly created) fork.
I've divided the "building-guide" into several pages, trying to keep the "common" parts away from the IDE-specific parts.