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This is an app you’ll find on one of your home screens, represented by a gray cog icon. If you don’t see the icon, check the Utilities folder.  It’s in the 5th section. If Restrictions are already enabled, you won’t see this button. Instead, you’ll be at a menu titled “Restrictions.” Skip to step 7, and enter your 4-digit passcode when prompted. This is a passcode that you’ll use when you want to make changes to blocked websites on this iPhone. If you lose or forget this passcode, you won’t be able to edit restrictions without erasing the content from your iPhone.  It’s at the bottom of the first section. ” This is the final option, at the bottom of the screen. Use the format www.sitetoblock.com. Some sites have mobile versions that you’ll need to block separately. For example, Google’s mobile website is accessible at m.google.com. The website you entered is no longer accessible in Safari.  To block another website, tap Add a Website again. If you’d like to block all adult/obscene content, scroll back up and tap Limit Adult Content.

Summary:
Open your iPhone’s Settings. Tap General. Tap Restrictions. Tap Enable Restrictions. Type a 4-digit passcode. Type the passcode again to confirm. Tap Websites. Scroll down and tap Add a Website under “Never Allow. Type the URL of a site you want to block. Tap Done.