Place the rose petals in a food processor or blender along with the lemon juice and ¾ cup of water. Blend for about 1 minute, until the mixture is very smooth, then gradually add the sugar in a slow stream as the blades spin. Continue to let the mixture blend for another minute or two to help dissolve the sugar.
Separately, in a medium saucepan over medium-high heat, combine the pectin with ¾ cup water and let it boil for one minute, stirring constantly.
Turn the food processor on again and slowly pour in the hot pectin. Let the food processor run for about one minute. The mixture will become foamy and lighter in color.
Immediately pour the mixture into the prepared jars with lids loosely secured. Cool in the refrigerator or leave out at room temperature until it has jelled. This jelly will store in the refrigerator for up to 1 month or in a freezer-safe container for 6 months. Alternatively, you can process the rose petal jelly in a hot-water bath for 10 minutes. It is best when consumed within 12 months of canning.