It's very important to pick birch fungus correctly. All fungi are different, as you would expect. To make tinder, you should pick only fungi which grow on dead birch trees. It's also necessary to take from the body of the fungus precisely the part that will become your tinder. This part consists of a velvety material that is pleasant to the touch and is often called "forest suede". It is found between the light, firm upper layer of the fungus and its spores. The piece of fungus obtained for its future use as tinder should be placed in a solution of lye, which can be made by mixing cinders from any hardwood species of tree with water in a 1:1 ratio. The resultant slurry can be left to infuse or can be boiled thoroughly, and then it is mandatory that you strain it. The tinder should be left to soak in the lye for 2-3 days, after which it should be taken out and dried. During the drying process the tinder should be kneaded from time to time. The dried tinder can be beaten with a wooden hammer. If you did everything correctly, then you should have ended up with a velvety soft piece of "forest suede" which will easily catch even the smallest sparks and thus begin to smoulder intensely.