Pressing flowers is a relatively straightforward process. You simply choose your flowers and lay them carefully in an unneeded book. (I find phone books are ideal. They are big, thick, have extremely absorbent pages, and keep arriving at my door.) You then close the book and place something heavy on top. The pages of the book absorb moisture from the flowers, while the weight flattens it. (Some flowers, such as thistles, are so thick they will never become completely flat. Choose relatively thin flowers for this project. For variety, you may want to press bits of greenery as well.) Leave the book alone for a couple of months, and when you come back, carefully open to one of the pages with flowers on it. If the flowers have been completely flattened and preserved, you are ready to proceed to the next step.
To make the bookmarks, you will need:
Acid free paper
Archival glue stick
Carefully remove the flowers from the book, and lay them in a single layer on a piece of white paper. Seeing the shape of your flowers will help you decide the shape of your bookmark. Do you have large, round flowers? Highlight the prettiest ones by making each the center of a square bookmark. Did you dry your flowers with their stems? Emphasize the length by making long rectangles. Experiment with shapes, such as elongated hearts.
Use acid free paper as the base of the bookmark, to avoid later discoloration. The stiff sheets sold for scrapbooking works well for this project. Choose solid colors, or those with a small all-over pattern or a subtle stripe. Look for papers that will complement the colors of the center of the flowers, or that contrast with the color of your main flowers. Avoid papers that have too much texture, as it will make it harder to laminate.
Once you have cut your shapes, lay out all the flowers for your design. This allows you to move them around, or change them out for ones that better fit your composition. When you are happy with the design, use a toothpick to carefully pick small globs of archival glue from the surface of your glue stick (make sure you use archival-quality glue, available in the scrapbooking section, to again to avoid discoloration) and place a single glob on the back of each flower. Flip the flower back right side up in place in the design, and press gently to spread the glue. This method reduces the likelihood of tearing a flower compared to rubbing the flower across the glue stick. You are just tacking the flowers down, so don't worry if the edges are not covered.
When you have glued all the flowers, place the bookmark on a flat surface and put a weight on it (again, the trusty telephone book comes in handy) and leave it overnight. This will further flatten the glue, and your artwork.
The next day, remove the weight. Cut two pieces of lamination generously larger than your bookmark. Remove the paper from one piece of lamination. Carefully lay the bookmark on it. You only get one shot at this, so make sure the lamination is flat and that you place the bookmark away from the edge. Remove the paper from the other piece of lamination. Press it flat onto the surface of the bookmark, rubbing your hands across the lamination to remove any bubbles. Trim out the bookmark, leaving at least a half inch margin of lamination.
If you have access to a heat laminator, the process becomes simpler, and the results often more professional looking. You just make sure your bookmark is as flat as possible, then run it through the machine, following the manufacturer's directions (You may find you get less crinkles if you put larger flowers through stem end first). Office supply stores often offer professional lamination services. Some people choose to decoupage the flowers to the surface of the paper, and skip the lamination process altogether, but this results in a much more fragile finished bookmark, and should only be attempted with truly flat flowers.
Bookmarks make great gifts. Any time the recipient pauses while reading a favorite book, that person will think of you. They are also great conversation starters, allowing you to have your flowers on hand to help tell the story of your garden.