In short, ignore them... for actual details, you can google "Cry it out". There are a variety of techniques, varying from letting them cry for 5 minutes, to just letting them cry. Most modern ideas lean towards the 5 minute route, starting when they are close to 6 months old, as then they are old enough to not need feeding overnight.

It does work. Kids do learn that crying doesn't bring any attention, so they stop, and learn a sleeping routine. And with 7 kids, I can understand the appeal. With less kids, we went another route. When my kids were little, there was research saying these techniques were not healthy, but in the last 10 years newer studies have said they are OK. But it is a heated debate among parents.

Ultimately, every parent needs to make their own choice.