Summarize:

If there is a pocket on the left side of shirt, this will make things easy--you'll want to pin the flower through the top middle of the pocket, where it may be slightly reinforced and a little stronger. If there is no pocket, you will pin through the upper left side of the shirt. You can imagine pinning it above the person's heart or where a pocket would be located. Because the pin will be visible, you may want to choose a fancy gold pin or one with a decorative head. It will appear as though you are pinning the fabric together. Push the pin completely through this fold of fabric. Unlike on a lapel, the pin should be held horizontally, or parallel to the floor. With this method you are pinning the stem tightly against the chest and not actually piercing the stem of the boutonniere with the pin. The pin should be over the stem and holding it against the fabric. You want the pin to cross the stem near the top, where it meets the petals of the flower. Make sure the boutonniere is facing the right direction, with the flower facing toward you and away from the person wearing the boutonniere. It should again be as though you are pinning together the fabric pinched between your fingers. Imagine that the stem of the boutonniere is in a valley and the two folds of fabric that you pinned on either side are mountains. When you look at the pin, it should go into the shirt, then out, then over the boutonniere stem, then into the shirt and then out a final time. The shirt should be smooth and the flower secure.
Figure out where to position the boutonniere. Pinch the fabric of the shirt between your fingers of your non-dominant hand and slide the pin through with your dominant hand. Slide the stem of the boutonniere between the shirt and the pin. Pinch the unpinned fabric on the other side of the boutonniere stem and push the pin all the way through. Stretch the shirt so that the fabric you pinned together lays flat against the chest.