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The airside shuttle bus that United provides is only available to domestic transfers. International arrivals go out landside after customs and must go through security to take a connecting flight.
Here is what will happen when you arrive at Newark:
You will first go to US immigration, use a kiosk or have an interview, then go to baggage claim and pick up your checked bags. You then take these to customs where you either get your baggage inspected or get passed through.
The bag drop is right after you pass customs. It shouldn't be hard to find, as everyone else will be dropping their bags there too.
After you drop your bags, you'll be landside in Terminal B. You'll have to take the AirTrain to Terminal C, then go through TSA security, and finally to your departure gate.