One should know that a fresh-flower bouquet is a living cutflower, which has been separated from its root system and thus the roots are no longer guaranteed and not all flowers are treated the same.
Cut flowers should first be cut at the bottom end with a slanting cut before they enter the vase (lukewarm water!), So the water can be absorbed very well. Wood-like stems are preferably cut with a cross.
The flower care in sachet form (which in most cases are given) are quite useful and should not be thrown away. This keeps the flowers longer in the flower. But do not forget: the area in the vase should be freed from the leaf so that no premature mold is formed and the bacteria do not attack the water.
If you place your flowers in a cool place at night, extend their life.