Generally you gather them in the fall. You want freshly fallen ones. Dried and not at all green.
If they're from last fall/ winter they're probably not much good, as the snow and rain has caused the good stuff in them to seep out.
other than that, you can toss a couple in the water, or to tan the water you take a gallon jug or something, add some conditioned water, smash up about 10 leaves, and let that sit 24hours, then strain off the leaves.