These are all natural resources found in Normandy.
(please pardon the French comments on the images, they do come from the original article writtten in French)
The marcasite can be found on the flat under the cliffs at low tide. Strangely, it is easier to find after strong west (sea) winds. It is needed to have a close look as they can be mistaken for rounded stones. The general aspect is that of bawls, or agglomerated bawls, of a dark colour, so they are somehow easy to find.
Flint is also easy to find, most rollers in High Normandy, are made of flint.
Tinder fungus (amadou, fomes fomentarius) is also gathered during forest hikes and grows as a parasite on dead or living trees.
The process :
Break a flint in order to obtain a sharp tip, break the marcasite, and create a channel, the channel will allow to direct the sparks.
With the tinder fungus (needs to be lenghtly dried), the higher down like part is used and scraped, then spread over a large surface, maximising chances of catching a spark.
The sharpened flint is then used to strike the marcasite, in order to throw sparks on the tinder fungus. Only one spark is enough to succeed, then it is needed to blow it into a nice ember taking in the amadou.
Once the ember is taken, it can be moved to the nest, or bundle of grass, placed in a central position, and after blowing it increasingly stronger, a flame will appear !
Jean-Baptiste, traduced to English by James.