I have preserved flowers and herbs for many different projects over the years. There are several different ways to accomplish preserving flowers and herbs, but what it really boils down to is what kind of project you will do with the end dried herb or flower that matters.
Each technique is a little different. The details below will give you an idea of which method you may want to use to preserve your own herbs and flowers.
Air Drying: Air drying is the simplest of the methods and of course, the oldest way to dry your plantings. Gather up and bunch with a rubber band, hang them in a dry and warm place. Most of my items take several weeks to completely dry out. After they are completely dry, wrap them in tissue paper and store them in a box to take out as needed for projects.
The only cons that I will add to this method is the fact that your colors fade or turn much darker than your original specimen. If you want true colors, this method may not be the best.
Drying Using The Microwave: If you have a small flower or herb batch to dry, the microwave works very well. Keep in mind that the items do not hold their shape very well, so you might want to use it for just drying herbs.
Place an absorbent paper towel on top of a glass turn table inside of the microwave. Place your herbs on the paper toweling and close the door. You will need to experiment with times and temps. To start out try to set your microwave to a medium temp and set your time to 1 minute 30 seconds. If the herbs are not dry, add an additional 30 seconds and check again. Continue until completely dry. Store in a covered container.
Using Silica Gel Crystals: You can dry flower heads for potpourri or to use in flower arrangements in silica gel crystals. After completely dry you can add a wire and use floral tape to complete a stem.
Silica gel crystals can be purchased in floral shops and garden center. They are a little more expensive but can be used over and over again. They remove moisture from the plant while keeping the color and structure.
To use the gel crystals pour half of them into a plastic container. Bury your plant into the sands, spooning the mixture all around the plant, until the plant is completely covered. Place a lid on the container and leave sit a week to dry out.
Once your flowers are dry, spray them with some light polyurethane varnish to keep for years of enjoyment.
Drying Petals and Small Leaves: Place them on a wire baking rack. Allow to dry in a warm, dry place. Once dried out you can store them in an air tight container.
I find this process extremely beneficial for drying small flowers to make cards and sayings to frame. I spray my final project with a clear coat to keep the flowers from picking up moisture.
Pressing Flowers: Purchase a flower press from any craft shop. A press consists of two pieces of wood with a hole and threaded bolt with a nut in each corner. The press comes with 20 blotting papers and two pieces of cardboard. Throw out the cardboard inserts and replace them with newspaper.
Open the press and place newspaper on the bottom, then a blotting sheet on top. Place the flowers on the blotting paper leaving space between flowers. Place another layer of blotting paper on top of the flowers and then more newspaper. Continue to build up your layers in the same manner. Screw down your press tightly and place it in a warm, dry place for 6 weeks.
The best flower and leaves for pressing are flowers that are simple and not thick centered. The flowers and leaves should be new, fresh and picked during the mid-morning on a dry day.