How to Enable Family Sharing in iOS 11

Family Sharing in your iOS 11 device allows you to share the purchases you made using your Apple ID among all your family members. When enabled, the family members that you added can download all your purchased apps on their iOS devices without re-purchasing them or accessing your ID.
In addition to this, every time any of your family members makes a purchase from App Store, a confirmation notification is sent to you before the payment is made. You can accept or deny the purchase request as needed.
However, before you enable Family Sharing on your iOS device, you must add a payment method. Here’s how you can set up and use Family Sharing on your iOS 11 device:
Add a Payment Method
    Get to the Apple ID window
    Tap Settings, and tap your name from the top of the window that opens. This brings up the Apple ID window that contains various options to configure your Apple ID, iCloud, iTunes & App Store settings, and manage your other Apple devices.

    Get to the Payment & Shipping window
    Tap Payment & Shipping, provide the password for your Apple ID, and tap OK. This confirms that you’re the correct person to manage the payment methods and shipping address on your iOS device, and takes you to the Payment & Shipping window.

    Initialize the payment method setup
    Tap None, tap Credit or Debit Card on the Payment Type window, and tap Next. This tells your iOS device that you’re about to add a debit or credit card as your payment type, and takes you to the Payment Details window.

    Add your card number
    Provide your card number under the BILLING INFO section and tap Next. This checks and verifies the availability of your credit or debit card, and takes you to the next window.

    Provide your card’s expiration date
    Tap the Expires field, and use the calendar to specify the expiration date of your card. Doing so confirms the validity of your card upon completion.

    Provide your card’s CVV number
    Tap inside the CVV field, type the CVV number present at the back of your credit/debit card, and tap Save. This confirms the authenticity of your credit/debit card, and adds the card as your preferred payment type.

    Get back to Apple ID window

Tap the Apple ID option from the top-left. This takes you to the Apple ID window from where you can manage your other ID related settings, including Family Sharing.

Set Up Family Sharing
    Get started with Family Sharing setup
    Tap Set Up Family Sharing > Get Started > iTunes & App Store Purchases. This tells the device that you’re about to set up Family Sharing for iTunes and App Store, and are likely to make purchases, or approve or decline the purchase requests by your family members. You can even choose other options to share with your family as needed.

    Confirm and share payments from your account
    Tap Continue on the Confirm Account window, and tap Continue again on the Shared Payment window. The Continue button on the Confirm Account window confirms the account you want to use to make purchases, and the Continue button on the Shared Payment window confirms that the payment method is associated with the current account.

    Send family sharing invitation
    Tap Invite Family Members, tap the + icon from the top, select a contact from the list, and tap Send. This sends an invitation to your selected contact to be added as your family member to your Apple ID. Optionally you can type a text message before sending the invitation. Once your contact accepts your invitation, they can use the apps you purchased using your account, and you can accept or decline their purchase requests.

    Tap Done upon completion