How do I open my mobile app to approve my transaction ?

Enabling notifications makes this process easier. If you've enabled notifications, you'll be taken to the mobile app log in. If notifications are not enabled, launch your mobile app as you would normally.

When you open the app, the "Approve a Transaction" button will be at the top of the screen, above your accounts.

For how to use the 'Approve a Transaction', please see How does approving my transaction in my app work?