Harvest your rose petals when they first bloom because this is when they are the most fragrant. I know, I know, this will be hard! Remember, you only need a few blooms for the tea so you will still get to keep your rose bush mostly undisturbed.
If you do pick a few to store, you will need to dry them in a dehydrator or in an oven at a low temperature.
Another quick note: You will want to make sure that the store you buy your roses from did not spray them with chemicals.