Add the flower food to your vase water. Please follow the instructions on the sachet.
Condition your flowers & foliage. This is a fancy way of saying remove any leaves which will end up below the water line of your vase. You basically don't want any leaves left underwater because they will rot and cause a build-up of bacteria. These bacteria will not only pong, it will block the stem ends, preventing the uptake of water. I have conditioned your flowers for you so you won't have to do this unless you are cutting the stems down a lot to suit a small vase.
Cut the stems by 1-2 inches with a sharp pair of scissors at a 45 degree angle. Make sure you don't crush the stem ends while doing this otherwise you will damage the water-conducting tissues.
Place the stems in the water/flower food mix straight away. You have to be quick otherwise air will move into the water-conducting tissues & plug the cells.
Keep the flowers in a cool spot. Flowers will dehydrate if they are placed in direct sunlight or are near heating or cooling vents. It's not a good idea to put them close to radiators or appliances which give off heat.
Change the water every 2-3 days. This may sound slightly annoying as it's an extra job to do around the house, but your flowers will benefit. If you don't, the flowers won't drink enough to survive so they will start dying, and bacteria will start building up in the vase water which will ultimately poison your blooms.