iPhone: starting with the iPhone XR and XS models, NFC is always active, no additional action is required. On iPhone models 7 through X, swipe to "Control Center" and press the NFC button. This action should be repeated every time you need to read the item.

Android: make sure that NFC is enabled. Easy to find by searching in the settings or swipe from top to bottom, the NFC button should be active.

iPhone has the NFC reader is at the top. You can tap the card to the back or to the front side of the phone.

Android could have the NFC reader either at the top or back center, depending on the model of the phone. Try to tap the card to the back center or top front side of the phone.

To activate the item, you need to create an account or log in to an existing account.
You can sign up using your Google, Facebook account or Email.

That's it! Your item is now activated.