Fats and oils extract the oily and resinous properties of a herb, and these are often the antibacterial, antifungal and wound-healing components we really want.
Choose your leaves or flowers. Don’t wash them before use – not only will this destroy some of their delicate oils, but as soon as you introduce water into the mixture, you are setting it up to develop mould. (One of my friends was complaining that his nettle oil had an unpleasant odour, which it really shouldn’t, nettle has a fresh, rather lovely scent as an oil. I discovered that he had been rinsing the herbs before packing them in the jar.) To avoid this, make sure you pick your herbs on a dry, sunny day, and pick them from an unpolluted location
To make your cold infused oil, take your fresh herbs, cut them up, pack them tightly into a glass jar, and cover with vegetable oil (olive, sunflower etc.). Leave on a sunny windowsill for 2 weeks, shaking daily. Strain into a clean, dry jar, label and keep in a cool, dark place for up to a year.
Infused herbal oils may be used as they are or thickened into salves with beeswax. Unlike essential oils, they do not need to be diluted for use.
Source: Making Cold Infused Oils