I will be the first to say, I am no expert and this is something I have recently gotten in to. I have failed at this a few times and have learned from those experiences. I have definitely improved since I first started, and have learned some new techniques! There are many techniques friends have shared with me, as well as some I've found online.
My absolute favourite way to press flowers can take about two weeks, so if you're looking for something quick this may not be for you. Do you remember those THICK phone books we used to get every year? If you happen to have one tucked away in your basement, it is time to dig it out! If you don't have one, that's okay - any book with lots of pages works great. CAUTION: flowers can transfer colour, sap, and more. DO NOT use a book you care about!
Now that you have your book, grab some waxed tissue paper! If you don't have any, you can pick some up at a dollar store, or a craft store. I cut mine into smaller pieces that fit to the pages of my book. This prevents the flowers you are pressing from sticking to the pages and/or growing mould. Ensure you have tissue paper on both sides of the flower.
I like to leave my flowers out of water and let them sit on the counter for 24 hours before I press them. This allows them to start drying out, and lose some water. I find they also become more flimsy, giving me an easier time when I need to close the book. Place your flowers inside between the tissue paper and close your book!
For 14 days leave the book closed and either put large rubber bands around the book, or place something heavy on top. No peeking! Patience is key. Ensure it will not be disturbed for the full period of time. You can then remove the flowers and use them as you please. I have found that the flowers do tend to lose colour and turn brown when I press them, at this point I am unsure how to combat this issue!
Once you have waited your 14 days, carefully open the book and slide your pressed flowers off the tissue paper. Sometimes they can stick a bit, gently peel the blooms off the tissue. I keep mine in organizational drawers to ensure they stay intact! I like to use mine to make hand-made cards. You can see my cards here, or scroll to the bottom to see my gallery! Uses also include, but are not limited to, gluing to canvas or mason jars, keeping in a shadow box, or gluing to a photo frame. Happy pressing!