Summarize the following:
Tap the Yahoo Mail app icon, which resembles an envelope on a purple background. Your inbox will open if you're logged into Yahoo. If you aren't logged in, enter your email address and password before proceeding. Tap and hold an email that you want to mark as spam. A checkmark will appear next to it. It's in the bottom-right corner of the screen. A pop-up menu will appear. This option is in the pop-up menu. Doing so moves the selected email to the Spam folder. It's in the top-left corner of the screen. A menu will appear. Scroll down to the Spam folder, then tap the trash can icon to the right of it. This will confirm your decision and delete the Spam folder's contents.

Summary:
Open Yahoo. Select an email. Tap ⋯. Tap Mark as spam. Tap ☰. Clear the Spam folder. Tap OK when prompted.