Just before they touch a spark will be produced which you must catch on your tinder. Soaking the tinder in a flammable liquid will help this along, although care should be taken. This is a friction method that requires a lot of practice but can be surprisingly useful. It consists of a hardwood spindle being rotated on a softwood base. This will first produce tinder, and then will ignite the tinder.
Useful woods are Balsa, pine, bamboo for the soft wood.
Oak, ash and beech for the hard wood. They must all be perfectly dry. Make an even, cylindrical spindle from the hardwood. You may also find it helpful to have a block of stone or wood on which you can push down the spindle. First you wind the bowstring around the spindle.