Tap the Yahoo Mail app icon, which resembles a purple box with a white envelope and "Yahoo!" written on it. If you aren't already signed in, enter your email address and password and tap Sign in. It's in the top left corner of the screen. This option is near the top of the menu. It's an option below your primary account name. This link is at the bottom of the screen. You'll need to fill in the fields on this page with the following information:  First and last name New email address New email password Phone number Birth date Gender (optional) It's at the bottom of the screen. Doing so will prompt Yahoo to send a text message to your provided number in order to verify your account. You can also tap Call me with an Account Key if your plan doesn't allow for texting. It will be in your phone's Messages app; the message will be from a six-digit phone number, and it will say "[number] is Your Yahoo Account Key." Make sure you don't close the Yahoo app when you do this. You'll enter the five-digit code in the text field in the middle of the screen. It's at the bottom of the screen. If your code matches the one Yahoo sent you, your account will be created. You now have a new Yahoo email address in addition to your primary one.
Open Yahoo Mail. Tap ☰. Tap Manage Accounts. Tap ＋ Add Account. Tap Sign up. Enter your new account information. Tap Continue. Tap Text me an Account Key. Open the message from Yahoo. Type in your code. Tap Verify. Tap Let's get started to proceed to your account.