Just place an orange PokéStop pin on the map, take a snap of the spot you want to submit and surrounding area, add a title and bio describing the locale (along with some extra info for Niantic's reviewers) and hit submit. Then the team tasked with managing the Ingress Operational Project Recon -- a web app for crowdsourced portal uploads and edits, itself in private beta -- will review your nomination and tell you via email if it's eligible. Successful submissions will be transformed into PokéStops or even Gyms.
Before you start scouting locations, though, it's probably wise to peep the criteria. Niantic is after everything from places with history or educational value to public parks and libraries. And, as it notes at the bottom of its blog post, it can use your content beyond Pokémon Go on other apps and games (meaning it could make it on to that AR Map Niantic is building).
We've seen sponsored PokéStops in the past (from Sprint and Starbucks) as one way of monetising Pokémon Go. But this type of user-uploaded content is all about expanding the hit game's scope, writes CEO John Hanke: "Fostering these moments of discovery and exploration is the foundation of what we do. The world is a big place, and the only way we can find the very best locations for inclusion in our game is with the help of our players."