Put a couple of light colored plates in the freezer.
Using a large pot, add the raspberries and heat them until they simmer. As they come to a simmer, crush them with a potato masher.
Let the raspberries simmer for about 15 or 20 minutes. This will help get rid of extra liquid.
Add the sugar to the raspberries. Stir to dissolve the sugar.
Bring the mixture to a rapid boil.
Continue to cook the mixture while occasionally stirring it until it reaches 220 °F. For higher altitudes, 8 °F above the temperature that water boils is often used for a rule of thumb.
You can check for the jellying point using the plates from the freezer in addition to (or instead of) measuring the temperature. Take a plate from the freezer (from step 1) and spoon some of the jam onto the plate, when you tilt the plate you should see a slow decent. If it is runny then it is not ready. You can also push your finger into the jam and if it is at the jellying point you will see ridges form in the jam.
Let the mixture cool. Skim off any foam. Transfer the jam into hot, sterilized jars. Seal and process using a hot water bath. If you plan on using the jam in a few weeks and don't process them then seal the jars, cool, and refrigerate.