in a pinch i suppose you could pour hot water over petals, let it infuse, then strain it, and there. you will get rose water, but you'll also get a coloration ranging from pretty pink to murky yuck. it also goes bad very fast, within a few days. the alternative is a bit more elaborate, but totally doable. and it yields a clear liquid, perfumed with all the promise a rose carries, and a floral-sweet taste. while not shelf stable, it will keep in the fridge for about a month.
first get out the tallest stockpot you have. then put in a tall heatproof dish, like a ramekin or a souffle dish. put the rose petals all around it. you want 6-7 cups, they'll come higher up than the ramekin, so press them down gently. then start adding water, until the petals are loosely packed in it. you don't want them floating atop an ocean. the water must not be higher up than three quarters of the height of the ramekin. set the pot over medium heat. then get a glass bowl that will fit snugly on top of your stockpot, so you'll have a double boiler. no steam must escape. the bottom of the bowl must not touch the rim of the dish inside. fill the bowl halfway with ice. place it on top of the stockpot: you've created a still. bring the heat to medium high and watch how all the condensed rose water will drip back in the ramekin. don't remove bowl to see how much you have. don't let it boil more than 45 minutes to 1 hour, you'll only dilute the stuff. let cool, pour into clean bottles and refrigerate. if anyone knows of a non-chemical way to make it shelf stable, i'd appreciate the heads up.
if this post is alambicated, please leave a comment and i'll try to offer a clearer explanation.