How do I implement my own Theme repository?

I am in the process of developing a (single) theme that I will use for friends, families, clubs and customers. I do not want it shared via wordpress.org at this stage as I only want it used on the sites I develop. I may decide to release it to everyone later but at this stage I wish to keep its use to those sites I create.

So my question is how can I set-up a theme update repository on my own host/web site? In that if I make an update to the core theme all the web sites that use my theme get update notifications and they can be updated as if the theme were hosted on wordpress.org.

If there are more than one option to do this maybe suggest them all, provide links to info etc as I want to make it as simple as possible 🙂

Thanks for your help 🙂