Safari extensions follow the same process as Opera and Google Chrome. Safari includes an extension builder that make easy to create the extension, package it and test it. The documentation for developers is quite good.
However, the submission process is completely obscure. I submitted the plugin over four weeks ago and I have no idea what Apple did with it. I don't have any reference number or any way to track it. This may explain why this is the only browser with a 3rd party website dedicated to browser extensions. I plan to submit Zscaler Safe Shopping to this site shortly.