One pint of rose petals
I found recipes that ask for 2 pints of petals which is a vast amount unless you have a huge garden (or don't mind taking all the flowers from your garden at once), or perhaps 200g which is fine but the petals have to be dry when you weigh them or the measurement is skewed. One recipe told me that 15 roses would be 2 pints. I found that 15 of my roses made about 1 pint. So see what you get from your garden.
Wash the rose petals
½ pint water
Strain the liquid and add about 4 tablespoons of lemon juice. This will brighten the colour.
Remove from the heat and add 50ml liquid pectin.
At this stage one of the recipes I looked at suggested that you could add a few of the discarded rose petals. I thought not. My 25 years ago recipe left the petals in but I found that the jelly + petal mixture squeaked. Call me weird but I don't like food that squeaks, and a scone with clotted cream and rose petal jelly should be a great delicacy... and I don't want it to squeak. So I left the petals out. Besides, by this stage they had taken on an unpleasant dull mauve colour so I didn't fancy adding them back into the mix.
Your jelly should be ready to set. Take a little bit out with a teaspoon and drop it on to a cold plate. It should show signs of setting almost at once. If not, boil for a little longer, or maybe add a bit more pectin and try again. Basically, see how you go.
Rest for a couple of minutes and then pour into the sterilised jars.
The roses that I used are dark red or deep pink. If yours are a lighter colour you may feel the need to add some food colouring. Or not. The first rose petal jelly I saw was pale pink.