Mash (with a potato masher or if you don't want any chunks you can use a blender) and measure your fruit (dip peaches in boiling water for about 45 seconds to make peeling easier).
Mix raspberries and peaches together in a large pot. Sift in the pectin a little at a time while stirring.
Once all the pectin is mixed in, set a timer for 30 minutes and give the fruit a stir every 7-10 minutes.
After the 30 minutes, mix in the corn syrup and then place the pot on the stove over medium heat. Stir in the sugar a little at a time and cook for about 5 minutes until the sugar is dissolved (when jam reaches 100 degrees F).
Pour jam into clean plastic containers or glass jars.
Jam lasts 1-2 months in the fridge and up to 1 year in the freezer.