Schedule an in-person interview online or by calling (855) 347-8371. Note: these appointments take months to schedule and are often at airports' TSA offices, so consider booking your appointment a few hours before your next flight to save yourself an additional trip to the airport.
Bring your passport or a birth certificate and driving license to the interview. They will ask you questions about your application and scan your fingerprints (which you'll later use at Global Entry kiosks to identify yourself).
Once approved, add your Traveler ID to all your frequent flyer programs so it automatically ads TSA pre-check to your boarding passes.
Bonus tip: While children do need their own global entry accounts to skip customs, the rules are different for pre-check. Parents that have Global Entry and are traveling domestically can bring kids 12 and under through TSA pre-check lines.
Programs that Reimburse the Fee
As TPG mentions, the following credit cards have programs that reimburse the Global Entry $100 fee:
Centurion Cards (consumer and business)
Citi Thank You Prestige Card
Citi AAdvantage Executive World Elite MasterCard
Mercedes-Benz Platinum AmEx
United Mileage Plus Elite Members (Global Services, MileagePlus Premier 1K, Mileage Plus Premier Platinum levels)