How to set up Apple Pay Cash
In order to use the new feature, you'll first need to upgrade to iOS 11.2. To do this, launch the Settings app > General > Software Update to download and install. The update is gradually rolling out to users this week, so if it doesn't appear available to you quite yet, check back again a couple hours later or even the next day.
Once you're running 11.2, launch the Settings app again and select Wallet & Apple Pay. Turn on Apple Pay Cash, and tap Apple Pay Cash under Cards. This will walk you through a series of prompts to verify your identity and allow you to tether your account to your real-life bank account. Your Apple Pay Cash card will function essentially like a digital bank account where you can "bank" received funds, use them to pay others, and "cash out," much like how you would using Venmo.
Note: you may receive an error message in the midst of setup saying "Apple Pay services are currently unavailable." This is likely due to the fact that Apple is very slowly rolling out the new service. iOS 11.2 was reportedly not initially meant to release this soon -- it was allegedly pushed out early and urgently this past weekend to patch a serious glitch affecting iPhones and iPads, and Apple is waiting for a few more days to switch on the service for all users.