Sterilize jars and keep them hot in the canning pot.
Put the clean raspberries in a wide 6-to 8-quart pot. Mash the raspberries and add the sugar. Bring mixture to a boil over high heat. Stirring occasionally, cook the mixture until it registers 240 degrees F on a candy thermometer. This can take anywhere from 10-30 minutes.
Remove from heat.
Ladle boiling water from the canning pot into a bowl with the lids. Using a jar lifter, remove the sterilized jars from the canning pot and pour the water back into the pot. Place the hot jar on a folded towel and drain the jar lids.
Ladle the hot jam into the prepared jars. Leave about 1/4 inch headspace at the top. Use damp towel to wipe the rims of the jar before putting a flat lid and ring on each jar. Seal the jar just until its finger tight. Do not twist on the ring too tight. There needs to be enough space to for the air to escape the jar during processing.
Return the jars to the water in the canning pot. The water needs to cover the jars by at 1 inch (more is better). Bring to a boil and let the jars boil for 5 minutes to process.
Remove jars and place on a folded towel. Let the jars rest undisturbed overnight. After 30 minutes, check that the lids have sealed by pressing down on the center. If it can be pushed down, immediately refrigerate the jars because they have sealed.