The screenshots are taken with iOS 7, however these instructions are the same for other versions of Safari on iOS.
Tap the "Settings" icon on the home screen
Return to the Home Screen by pressing the round button underneath the screen. You will see the grid of icons. Then, find the "Settings" icon and tap on it.
Scroll the Settings page down to the bottom
The Settings screen is longer than the actual screen of the iPhone or iPad, so to reach the "Safari" section you will need to scroll the screen down a bit by dragging on the screen with your finger.
Tap the "Safari" menu option
Scroll the "Safari" settings section down
Once again, the section you need to go to is further down the screen; drag the screen up with your finger until you see the "Advanced" menu option.
Tap on the "Advanced" menu item