I have 3 lavender plants in my yard (two french and one english) They have grown large over the years and they also grow like weeds - you can cut them way back and they will respond with growth. I love the smell, and since I have such an abundance I have made sachets for myself, and to give away as gifts.
cut the blooms when they are nicely purple and full. Cut either early in the morning or before sunset. The point being to cut in the cool of the day so that you and the bees don't have to enter into a discussions as to ownership.
Put the stems together and you can either tie them and hang the stems upside down to dry or I usually just take the whole handful and place them upside down in a jar to dry.That way when it dries, the lavender falls right into the jar. I have been known to put them in my car to dry and giving my car that wonderful fragrance.
Once they have dried ( you can feel that the blossoms are no longer soft and squishy) slide your thumb and index finger down the stems to strip them of the flowers. (I sometimes toss the stems on the floor to vacuum up - putting the scent in my vacuum) The whole room will smell like lavender, so if there is someone in your house who doesn't like the smell - do this outside.
I found a piece of sheer cloth in my remnants and I took it to the TechShop http://www.techshop.ws and using the embroidery machine there put a simple design (choosing one from those preloaded on the machine) to pretty-up my sachet.
I then folded the fabric and cut it out the size I wanted and finished off the two side seams