You can cut them short as in the picture below or cut tall branches (2 feet or so) which looks fantastic in a bay window — but there is no rule really. If you do cut tall branches, you may want to tie your bouquet (with garden or kitchen twine). You will be able to structure your bouquet more easily that way. Also, it is preferable to remove all flowers that will be below the water line. So next time you go on a walk outside, bring your garden scissor with you and happy pruning — your home is your canvass, enjoy!