To be honest, I've tried to extract the oils from St. Johns Wort, I do know it needs to be infused and probably should be bought in your health food store.
The flowers and 3 to 4 inches of the leafy stem are picked at midsummer for medicinal use. To make tea you just pour 1 cup of boiling water over 2 teaspoons of fresh, flowering tops. Steep for 10 minutes and strain.