Cut a piece of floral foam so that it fits snugly and protrudes a little from the top, should you wish to take your teapot on a journey (perhaps a vintage steam train) you might want to use a frog and floral tack for a belt and braces approach.
Prop up the lid by spearing the floral foam with some sort of stick, a cocktail one would be good, or just the end of a rose stem.
Then simply fill with roses, use different sized heads for a more natural look. We have used Hypnose, Cream sensation, and some snipped from an indoor plant.
Repeat as required, not forgetting to leave at least one teapot for actual tea making purposes.
Should you run out of teapots, you could turn your attention to storage jars