Many herbs are considered magical and are often infused into oils for ease of use. I've been doing some research into this and have come up with how to make infused oils easy and convenient in your home kitchen. So get witchy!
The approximate ratio of herb to oil should be about 3 tbsp herbs to cup of oil. There are two ways to do this. You can use a crock pot on low, or you can use a stainless steel mixing bowl above a double boiler. You don't want the oil to get too hot, so it's important to keep your eye on the infusion, so it doesn't turn rancid.
Mix your herb and your oil - I suggest something very light in taste and smell (in other words, no extra virgin olive oil, unless it's called for). I like safflower, but canola works too. Set mixing bowl over water (or pour into crock pot). Let heat/simmer for about 30 minutes (the crock pot may take a bit longer). Once the mixture is finished, strain through cheesecloth to remove the pieces of herb (again, this keeps the oil from turning rancid too quickly). If you're using the oil for cooking or decoration, it's fine to leave the herb material in and just bottle the oil. Store in airtight glass container (do not leave in direct sunlight as this will continue to reheat the oil, allowing it to spoil sooner).
Here are some websites you may find helpful for your magical plot or story: