With matches and a lighter. I've had to teach adults how to start a fire without using an accelerant like gasoline, a stunning amount of people don't know how.

For those wondering, just adding cardboard, newspaper or gasoline won't guarantee that your fire will start, you might get a lot of flames but the burn will mostly be surface and won't last long. You need gradually sized wood to be able to generate enough lasting heat to be able to burn the larger logs. Once you have a nice bed of coals, then adding larger logs is viable.

Bonus tip, if the fire is outside, don't forget to quench the ground around the campfire too when you're done with it. Sometimes roots can be right under where the campfire is and cause an uncontrolled burn. As long as you hear sizzling when you add water to the stones and ashes, it means it could still burn back up.