In my rental house I had a most gorgeous hydrangea... now I have a cutting from that plant full sized in the house I bought after that.
I took several cuttings, which all survived the process and the move between houses. You want fresh tops, just the top 2-4 leaves and about 2-4 inches of stem, under 6" total. I bought a small container of rooting hormone powder from the plant store and basically followed the directions on that package which I think were to stick the stem into the power then into very damp potting compound.
I kept the cuttings in containers for the first year I think, then I felt one set was strong enough to put in the ground but I kept another set back "just in case" but didn't need it. It's been about 20 years now -- the ground one is just starting to bloom for this year, I trimmed it pretty heavy this spring, this year will be fine but next year will be glorious!
When I think about it, I don't think I've ever had roots grow on flower cuttings I've had in water, and as we know those last a long time and have the opportunity to grow.