Almost everything that shortens a rose's life happens in the first hour after it reaches your kitchen. Here is the order we do it in, and why.
Cut under water, at an angle
A stem lifted out of water draws in air within seconds, and that plug of air is what stops it drinking. Cut two centimetres off with a knife — not scissors, which crush the channels — while the stem is submerged.
The sachet is not optional
It is sugar for food, acid to help the stem drink, and a little bleach to stop bacteria. Half a sachet in a litre, every time you change the water — not once on the first day and never again.
Change the water every second day and you will get eleven days out of a bouquet that most people throw away on the sixth.