Ask HN: Teach me to write 3rd party java script please?

Could someone please help me understand everything I would want to know in order to create third-party JS widget, like twitter embeds or facebook ads, in the shortest amount of time?

I am okay getting 80% of the value in 20% of the time, for now, and would love to know of any guides, pitfalls, and best practices for writing such widgets.

Basically, I will consume anything you're willing to throw my way so I could get from 0 to 1 asap. Appreciate it!