I dried a few flowers as an experiment this year. I’ve learnt that you need:
a proper place to hang them (the hall coat pegs won’t do as they get flattened)
strong colours (white flowers come out brown)
flowers cant be too dense to begin with (dahlias went mouldy in the middle).
They’re also really fiddly to work and break easily.
The best poppies to grow for their seed heads are in the Somniferum family - they produce lovely big pods that look great in autumnal arrangements.