Depending on the size-I use about 5-10 per 1g of water and let them steep for several hours until the water turns amber in color.
I fill my 5g rain water buckets about half full of leaves outside and let them heat up in the rainwater to use for some of my more sensitive CT tanks and for the Neon tetra when I spawn them.
In the house I will pour hot water over them and add an air stone to get thing going faster and then strain into a clean 1g jug once cooled down.
I also use lots of rainwater for some of my CT Bettas and I use this rainwater/oak leaf water to spawn all my Bettas, I have really hard well water.....
*Note: when using anything natural collected, make sure that it has not been exposed to any chemicals, pesticides, and pollutions, I would never use anything collected from road sides or trees that are by a street due to the pollution from autos/cars and/or spraying of pesticide for mosquito etc.....This can kill the fish and residue can contaminate the tank.