I made flower essences this spring, and it was very gratifying. I really like to make my own "stuff" for health, ease, and ritual. Flower essences turned out to be REALLY easy... just harvest the flower (preferably JUST the flower, no stem or bugs), I tried to choose especially healthy and unblemished ones.
These were some of the first ones, when I was putting them in bowls full of shallow water and covering them with plastic to keep debris out. I didn't love using plastic, even though it never touched the flowers or water... I'm sure ya'll understand... anyway, later I started using glass jars with lids.
It's good to harvest the flowers early in the day, put them in a small amount of water (ultimately, given that I was doing this for personal use, I didn't need much, less than a cup of water), and set them out in the sun to soak. The idea is that the essence of the flower will mix with the water, then they can be strained and bottled, and the energy of the plant is still there.
After everything sat in the sun for at least four hours, preferably longer, I filtered the water through a coffee filter, into dark glass dropper bottles that were half full of glycerine (food grade, available at health food stores).
These are the "mother essences" from which I can make smaller bottles of a single essence, or I can mix essences. For example, I made a sleep mixture for the kids of miner's lettuce, lavender, and borage. I should mention here that I did not follow published associations of the flowers. After I bottled them I sat and meditated with each one, and noting what came to me as the essence of that particular flower.
If you're interested in flower essences I got most of my information from the Bach Flower website (the makers of the infamous Rescue Remedy... love that stuff!, but these essences primarily come from flowers commonly found in Europe), and from the Flower Essence Society website (primarily United States flowers).