This is a really short guide on how to contribute to a Julia package.
Fork the original repository (probably the #master branch).
You may create a separate Julia environment for the project (see below).
Clone your forked repo: (activeProject) pkg> dev "link-to-your-forked-repo". It will show it's directory, which is a proper git clone of the repo. It will be something like C:/Users/cstamas/.julia/dev.
Modify the files as you want, commit the changes.
When you're ready, create a pull request on GitHub.
You can undo the dev command with free.
Here are the Julia community standards, and the contributing instructions for Julia. Also Katharine Hyatt's Making a first Julia pull request is a really good guide on contributing to Julia.