You will not be able to open your fuel door if your engine is on, so make sure to be parked at the nearest pump and to turn your engine off before pulling on the fuel door handle.
If you’re using a portable gas container instead, make sure you’re outdoors with a funnel and rag on hand. Gas is highly flammable, so you’ll want to use a funnel to avoid spilling gas on other parts of your vehicle. For the same reason, it’s also useful to have a rag at arm’s reach in case any spills occur on you—the last thing you want is a rash!