The old method: Submit and pray like mad you get lucky.

Thankfully the market is different now. You can actually "auction" your books if you're a successful indie author these days, getting an agent to contact their people and put forth a starting bid for publishers to compete for. And publishers bite, too, though they hate the cost, because it's one of the ways to keep someone at least "hybrid" publishing instead of going straight indie.

That said if you're brand new and trying to break in, you'll need some sales numbers for them to pay attention, an inside contact (who you know in publishing is far more important that what you write), or just a lot of luck to get your manuscript looked at.