Screw your jar's lid on tight and give the mix a good shake to get everything started. With St. John's Wort, you'll want to place your jar in a sunny window, or even outdoors in a place where the jar won't fall and break. [Some herbs are well suited to solar infusions, but others prefer a darker, cooler location - check your favorite herbal books to see what is recommended for the plant you're working with.] Leave the jar to infuse for 4-6 weeks, shaking it each day as you walk by. If you're working with St. John's Wort, you'll find that your oil will gradually turn a rich shade of red! This is supposed to happen. In fact, it is said that the darker and richer the red your oil yields, the higher the quality of it. Label your jar with the herb and menstruum you've chosen, along with the date and the expiration date of your menstruum.
After 4-6 weeks have passed, separate the plant material from the oil. I've found that the best way to do this is to line a potato ricer with a fine muslin cloth and press the oil out of the plant material into a large bowl. You could also line a fine mesh stainless steel strainer with muslin or cheesecloth and use a wooden spoon to work the oil out. Once finished, pour the oil into a fresh, clean jar and re-label it. Include the plant, the menstruum, the date it was infused, the date you strained it, and the expiration date of the menstruum. Once herbal oils have been strained, they are best kept in dark glass bottles in a cool, dark place away from sunlight and heat.