Put the 5 cups blackberries in a large pot and use a potato masher to mash them into a pulp.
Measure out the sugar into bowl and set aside.
Heat the berries on high.
By now, you should see juice from the blackberries. Add the sugar to the pot and stir well to dissolve.
Stirring constantly, add the pectin in a little at a time.
Bring the mixture to a full boil. Keep stirring! Boil for 2 minutes and remove from heat.
Spoon the blackberry jam into your prepared jars leaving 1/4 inch headspace. (see above for a link to instructions on how to do the water bath canning method!)
Place the lids and rings on. Make sure the bands are not too tight! They need to be just tight enough that they won't leak.
Place the jars in the canner and process for 10 minutes. There should be 1 inch of water above the jars.
After the 10 minute processing time, remove the jars from the water and set them on a towel to set for 12 hours.