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This is a fairly straightforward education but VERY important, especially as mobile data becomes more restrictive. This can be done in less than 5 steps!
1. Go to Settings -- Data Usage
2. Click on "Set Cellular Data Limit"
3. You will see two lines, the top indicating your monthly data threshold. The bottom line is your warning threshold.
4. Drag the top line to your desired GB amount (I prefer a little lower than my monthly data limit).
5. Drag the bottom line to your desired GB amount for a warning.
Boom. Done. I told you it was easy! One bonus tip: