You should never take the whole plant. You might also need to come back at different times of the year, eg. Place your specimens in a plastic bag so that they remain nice and flexible until you get them home or back to school. But if you leave them in plastic for too long, they will go mouldy. Timing is obviously important here.
Tweezers are a good idea here.
Smooth out leaves and petals. With plants that contain a lot of moisture, you might need to change the newspaper occasionally until the specimens are quite dry. Cut out a rectangular piece of white cardboard. The size is up to you. Arrange your plant specimens the way you want them.