1. The bigger pressure the better - it's obvious, isn't it. If you don't use at least a few encyclopedias flowers can contract or become wavy. Olny by applying enough pressure will you get good results. I strongly recommend using a press, not encyclopedia - it doesn't take much space and what's more you can regulate the pressure.2. The longer the better. It takes some time for flowers to get completely dry. For the thinest two weeks might be enough, other need one or two months. The main rule is that it's never too long.
4. Segregate flowers by thickness. Lay thin flowers on one sheet of paper, thicker and the thickest on the other two. Usually thickness is a fearute of every species, so you can easily segregate by species. For example, poppies should get a separate sheet and all kinds of pansies can be pressed together. In the press you can preserve big variety of blossoms by making 'layers' of flowers and paper.
In the first picture and those below you can see the final result.
You can use the same methods to preserve autumn leaves, but come on - autumn starts in October. We still should enjoy summer in the last days of September.
Have a beautiful day,