Attain a copy of BonaJson
The first part is quite simple. You need to download the code somewhere. Its available from github (link found in the project info) or though unitys asset store as a complete package. No matter how you acquire it, the source should always be availble for you.
If you acquired the package from the asset store, or from any any page supplying a .unitypackage file, everything should be set up already. BonaJson should now reside in /Assets/BonaJson.
You you downloaded the code from somewhere else or cloned the repository from github, make sure to add all the files and directories to your unity project. The proper place to add them is to /Assets/BonaJson, (though this is more of a convention than a requirement).
Start using BonaJson
No setup is required. The classes do reside in its own BonaJson namespace (this is not standard in other Unity plugins as far as I have seen) so namespace directives are needed. Please see the included examples for usage quick usage information. The complete library documentation is available at this page.