Burning is a chemical reaction which requires relatively much activation energy to start, but then produces enough energy to keep activating (i.e. sustaining) itself.

You just need to deliver enough activation energy to a given point on the material, which you can do with a flame, but also for example an electric spark, a glowing metal or a magnifying glass under sunlight.

You can also put the entire flammable material into an electric oven and slowly turn up the heat, until you reach this temperature: https://en.wikipedia.org/wiki/Autoignition_temperature