iOS 12 and up:
Step 01: Starting at the iOS Home Screen, click Settings.
Step 02: Click Screen Time
Step 03: Turn On Screen Time and press Continue after reading through the Screen Time informational window.
Step 04: Select “This is My iPhone”
Step 05: Click the “Use Screen Time Passcode” and enter a code when prompted.
Step 06: Click Content & Privacy Restrictions
Step 07: Enter your Restrictions Passcode to continue
Step 08: Turn the Content & Privacy Restrictions to ON position
Step 09: Select Allowed Apps and then toggle Safari to the OFF position (it will look like the screen below)
Return to the Home Screen to save these settings.
iOS 11 and older:
STEP 1: From your device's home screen, click the Settings app
STEP 2: Scroll down and select General
STEP 3: Select Restrictions
STEP 4: Enter (or create) your restrictions passcode
STEP 5: Switch the toggle next to Safari to the OFF position