1. First create an .ascx file with a placeholder.
2. Next, you will need to take advantage of registering your .js files in page's ClientScript. This is important because if you drop more than one instance of your control on the page, then you only want to include your .js file once.
3. Also, you will need to set all attributes that will be abstracted to the properties of the user control for ease of use.
4. Finally, you will need to write any initialization for each client instance. Also, you will need to put the content into your placeholder (notice the INamingContainer). Now here is how your OnLoad ought to look now.
That's it, enjoy!