What’s the best way to attribute indirect conversions from an ad?

You're referring to incrementality. 

This is the measure of the lift that can be attributed to a marketing campaign. Here's an intro as to how to figure it out:

Incrementality is a huge challenge for marketers. It's less than obvious, and calculating it requires dealing with complexity. It is also far from being 100% accurate. However, figuring out incrementality allows you to understand the connection between your marketing campaigns and the total growth of your product. That's an enormous win, especially when dealing with big budgets.