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While backfiring can be done manually with relative ease in old cars, it's important you have an understanding of what backfiring is, and what causes it. A misplaced spark or unexpected burst of fuel or air will cause a loud burst from the engine. While modern cars come equipped with systems to regulate these aspects via an Engine Control Unit (ECU), older cars (roughly before 1990) are far more malleable. It's important to keep in mind the reasons why regulating systems were installed in the first place. Excessive backfiring is very unhealthy for your vehicle, and can ultimately result in having to replace parts. Bring it to a steady rev. Prepare the vehicle as you regularly would. Normal safety checks (including dripping oil) are much more important here, as you'll be charging an open flame. The location in which you do this must be open, and relatively free of things that might get caught by the flame. This includes anyone who may be watching. Keep that at a healthy distance-- around 10 meters (33ft) should be good. This will prep your car for some backfiring. Ideally, you don't want to start moving fast while you're turning on the engine, so keep the pressure light. Keep your foot on the gas pedal as it starts up. Once it's up, press the accelerator down as hard as you can. This should cause the car to backfire.
Consider the reasons that cars backfire. Start your vehicle. Turn the engine off again, with your foot on the gas pedal. Wait a few seconds, then restart the car.