By the way, most replies (including mine) were centered around the 'where' (online, or pull). But since the title is 'how', I think it's a good opportunity to share this good talk from Nina Zakharenko. Most of it is really generic and language agnostic, and 90% of the talk will still be relevant to people not doing any Python at all.
How to be a good PR reviewer. And how to be a good PR submitter. (Spoiler; both could be hard, because of people, but nothing a walk or a coffee break couldn't resolve)