The active ingredients of this plant are captured by macerating the flowers and buds in several weeks in a vegetable oil, which then takes a beautiful yellow-orangish tint. It is however recommended not to apply St. John's Wort macerate prior to sun exposure (ideally only use it in the evenings), for hypericin makes the skin photosensitive. This preparation can be very useful for mild and sunburns. Amongst other emergency applications, it can help reduce muscle stiffness after sport: just massage it into your aching muscles and drink generous amounts of water.
The flowers and flower buds should be picked when they leave red-purple traces in your fingers, not before.
Select a glass jar, and fill it till half with fresh plucked flowers of St. John's Wort. Cover everything with an oil of your choice (almond or jojoba oil are great choices, as they are relatively stable). Close the jar and leave it for four weeks, next to a sunny window, regularly turning the jar upside down. Filter through a cotton gauze (like a simple compress), and put it in a new bottle.
Label the bottle, and voilà!