How do the boys start the fire?

They steal the glasses from one of the boys and use the lenses as a magnifying glass.

Jack seized Piggy's spectacles and Ralph then uses them as a burning glass to focus the Sun's rays and start a fire. Their intention is to build a signal fire, which can attract any passing ships or planes. However their over enthusiasm leads the boys to build an enormous bonfire which eventually collapses in on itself and releases a shower of sparks which set fire to a patch of jungle further down the mounatin.
Use Piggy's glasses.