Summarize:

It's a grey app with gears on it. You'll likely find Settings on the Home Screen. This option is near the bottom of the screen (iPhone) or in the upper-left side of the screen (iPad). It's near the middle of the General page. This is the passcode you used to enable restrictions on your iPhone or iPad. If you haven't yet enabled restrictions, first tap Enable Restrictions, then enter your desired passcode twice. It's the last entry beneath the "ALLOWED CONTENT" heading. You'll see a blue checkmark appear to the right of this option. This is the last option on the page. It's the blue button in the bottom-right corner of the keypad. Your selected site is now inaccessible in the Safari browser. This setting also applies to other popular mobile browsers such as Chrome and Firefox.
Open your iPhone's Settings . Scroll down and tap General. Scroll down and tap Restrictions. Enter your restrictions passcode. Scroll down and tap Websites. Tap Limit Adult Content. Tap Add a Website below the "NEVER ALLOW" heading. Tap Done.