I would start the seeds indoors near a window out of direct sun. Keep temperature & moisture consistent. You'll likely have to nurture them indoors until next spring when they can be acclimated to your garden. Good luck!

the link for the RHS gives advice on seed sowing. do you know which plants you have purchased seed for. the species will come true but if its seed from a named variety then they may not.

usually hydrangeas produce their seed in the autumn so the seed you have just bought is last years. it may not be viable sadly. but generally put the seed in a soil based compost like john innes no 1. bury the seed approximately the same depth as the seed is tall/wide. cover with grit and leave outside. try to make sure it never dries out. They usually need some winter chilling to help the seed break dormancy.

Thanks both of you. I am not entirely confident that these seeds will germinate but sometimes you have to take risks in the garden!