This is the process I go through when starting a fire with my firesteel and birch bark.
In the areas I travel birch is generally easy to come by and it makes for great tinder. There are other things you can use such as mashed up dry grass, fluffed up cedar bark, etc… but when I can find it I always grab the birch. Though please be kind and don’t take it from living trees. If you find a standing birch you should be able to find enough bark on the ground to serve your purpose.