How do studios decide which manga to make into an anime?
Producers look at what's currently selling and/or polling well and what gaps they have in their programming schedules, and they try to find a popular series that fits a hole in their offerings. Once they've decided what they want, (shounen anime, late night seinen anime, etc) they start looking for manga that fit the mold. If they can't find one, they either broaden their niche or start looking into creating an original program. If an anime company has ties to a manga publisher, they might be more likely to pick something from what they have, but ultimately it depends more on what they're looking for. Sometimes, if a manga publisher is really confident about the popularity of a series, they might go courting anime publishers, but that's relatively unusual.
TLDR: They go looking for popular series that fit the mold of what they already want to publish.