Although you can sometimes purchase herb-infused oils, they're actually very easy to make yourself. This is a folk-method of making comfrey-infused oil and its strength will vary depending on a number of factors. The time of year it is, when the plant material was picked, at what life-stage the leaves and plant are, how the leaves were dried, and how old the herbs and oil are.
If you grow or can forage comfrey yourself, pick the leaves when they're at their best on a bright, dry morning. Leaves should be young and lush and it's best to pick them before the plant flowers.
Dry the comfrey leaves completely. The stems are fleshy and wet so begin by pulling the leaves off and drying on a screen or in a food dehydrator. Alternatively, you can use needle and thread to string the leaves up like medicinal bunting. Air drying takes up to a week.
Fill the jar 2/3 full with dried* comfrey leaves. Tear or shred them to increase the surface area.
Fill the rest of the jar with a liquid oil of your choice such as sweet almond oil**.
Wait a minute or so and top up the jars with more oil if the level goes down. Also make sure that the herbs are completely submerged.
Place the jar(s) in a warm place for 3-6 weeks. The general advice is to give them a little shake every day. I tend to shake them when I remember to do it. You should pop the jars in brown paper bags if you decide to put them on a window sill***.
After the 3-6 weeks have passed, strain the leaves out of the oil by pouring it all out into a cheese cloth lined sieve/strainer. Capture the oil in a bowl below and make sure to wring as much oil out of the cheese cloth as possible. In most cases, the infused-oil color is different from the original. In the case of comfrey oil, it's far more golden.
Discard the herbs and bottle the infused oil up. You can use the same jar used for infusing or specialist dark jars that herbalists use****. Either way, make sure to store the oil someplace cook and out of direct sunlight.
The oil has a shelf-life of one year or the best-by date of the original bottle of oil you used. Whichever is closer.