Summarize:

It's the blue icon with a white bird. Log in with your account, if you haven't already done so. This will open the menu tab. You'll find this above the "Profile" option. This will leads you to the "Followers" list. Doing so takes you to their account page. This option is located at the top-right corner of the app. A confirmation message will pop-up on your screen, after doing so. This officially blocks your selected follower. You'll find this option on the top right side of your screen. Your follower should now be unblocked, but they will no longer be following your account.
Launch the Twitter app. Tap on your profile picture, at the top-left corner of the app. Tap on the Followers counting. Tap a follower you wish to block. Tap the button. Tap the Block option. Tap Block when prompted. Tap the red "Blocked" option. Tap YES in the resulting pop-up menu.