Now that Apple Pay Cash has arrived, you (hopefully) might end up getting sent some money right from inside an iMessage. If so, that money gets added to your Apple Pay Cash card in iOS 11.2, which you can use just like any other card in your digital wallet. If you're not so lucky, you can still add money to the card yourself without having to wait for a contact to send you money first.
It's actually quite simple to get started, but before you do, you'll need to make sure Apple Pay Cash is properly set up, which includes adding a debit card to Apple Pay (you cannot add money to your Apple Pay Cash card using a credit card).
Once you've enabled Apple Pay Cash, adding money to your card is simple. Open the Settings app, then tap "Wallet & Apple Pay." Under the Cards section, tap "Apple Pay Cash" to add to your balance.
Next, tap "Add Money." On the following screen, enter the dollar amount you'd like to add to your Apple Pay Cash card. Once you type in a number, you will see your adjusted Apple Pay Cash balance underneath that figure. The minimum amount you can load in one transaction is $10, while $3,000 in one transaction is the maximum. You cannot add more than $10,000 over a 7-day period.
When satisfied with your prospective balance, tap "Add" in the top-right corner of the display. Then, simply confirm the addition with Apple Pay on the final screen.
Apple will display the debit or prepaid card that will be used for this transaction in the box above To Apple Pay Cash Balance. If you have more than one card saved and would like to use a different one, tap that box. You can choose from all saved debit or prepaid cards here. Credit cards will be grayed out.
This new balance will be updated across compatible Apple devices attached to your Apple ID, so you can make Apple Pay Cash purchases with any of those devices; You can use an iPhone, iPad, Mac, or an Apple Watch, though you can't use a Mac for person-to-person payments.