If you want to remove the raspberry seeds, read the blog post for instructions. However, I don't do it, too much trouble.
Start by preparing your jars. I had enough jam to fill 4 large jars, if your using smaller you will probably need about 8. Wash and sterilize them, including the lids, and place them on a clean kitchen towel until ready to fill.
Clean and chop the rhubarb into small pieces.
Place the rhubarb in a very large pot, bring to a boil and cook for about 5 minutes or until soft, stirring from time to time.
In the meantime mix 100 g/ 3.5 oz of the sugar with the pectin in a jar. Place the lid on and shake very well to ensure that everything is mixed well. (Note)
Add the raspberries to the rhubarb pot and puree the mixture with an immersion blender. Add the sugar-pectin mixture, mix very well and bring to a boil while stirring continuously.
When it starts to boil, set the timer and cook for 3 minutes while stirring all the time.
Add the rest of the sugar, bring to a boil again and continue boiling for another 2 minutes while stirring all the time.
Transfer immediately to the prepared jars.
Can the jam if desired. Either in a canner or in a waterbath.
Make sure that you read the packet's instructions on how to use the pectin, the method might be different than the one I use. It is better to follow the packet's instructions if they differ from the ones I give.