Some methods I know of are exposing a wand to the light of a full moon over night. You can also hold the wand in your hands and transfer some of your own energy to it. I actually charged the Elder Wand I bought from Alivan's this way. Oak, with it's affinity for lightning, charges nicely during thunderstorms. I'm not saying go outside and point it at the sky during a storm, you're asking for a Darwin Award if you do. I'd recommend sitting near a window, or if it's safe, on a covered porch, and while watching the lightning [a favorite activity of mine regardless] , hold the wand and will it to charge during each flash, though it may not be necessary to will it to charge, indeed my oak wand self activated and started charging on it's own [I could feel it energize] .
Mine was taken from a branch that fell among many others during a very strong lightening / thunderstorm. I collected it the very next morning. It's a very old white oak, and oak has a tendency of being struck by lightening. The tree itself appeared to have been, not badly, but enough to be seen. So mine was naturally charged. I merely claimed it and smudged it. =)