To find flowers and other plants to press I usually look in abandoned fields or along the country roads. If those are hard to come by, make a trip to your local gardening store or flower shop and buy some!
To press the flowers you must use pressure. Pressure can come in many forms. As I have previously mentioned a heavy book like a dictionary can work. Stack bricks or other heavy objects on the dictionary for added pressure. Flower presses are hard to come by in stores. I would suggest buying one online or even making one yourself.
Place the flowers and plants in the heavy book or in the flower press using the pages or pieces of paper to separate the flowers. Once everything is in place store the book or press in a dark place for up to two weeks. In two weeks take the flowers and plants out of the dark and out of the book or press. Now you have beautiful flowers to do with what you will!
If you have any questions, feel free to comment.