Pour olive oil over the herbs until it is mostly full. Leave a little space for the flowers to expand once they have started soaking up the oil. However, if you leave too much air at the top there's a greater chance of oxidisation occurring, and this reduces the shelf life of the infusion.
Cover the glass jar with the lid. Leave it to infuse in a warm dry spot for about 3 weeks, stirring once a day for the first two weeks. If it is not strong enough you can infuse the same oil a second time around by adding more lavender/herbs to the oil and repeating the 3 week process. Once ready, strain the flowers/herbs well to remove the plant matter from the oil, and pour into the final glass bottle for use.
For a faster infusion method you can use heat.
The Heat Infusion Method
Use about 60g of dried herb, or 80g fresh herb per 300ml olive oil.
Place the oil and herbs in a double boiler or slow cooker on low with a lid on top. Allow the oil and herbs to infuse for 2 hours. If using a double boiler, ensure the water does not reach a boiling temperature. Stir the mixture every half hour.
Strain the mixture of plant matter, and repeat the process with more lavender/herbs if you want a stronger infusion. Once cooled, pour into a glass jar with a lid.