Put the rhubarb and sugar into your pan, mix and leave overnight. There will be a fair amount of slushy watery content in the morning. You might want to use a hand blender to mush down your rhubarb now, or you can wait until the mix is boiled, but at that stage, you would need to take much care, as you are dealing with boiling sugar.
Bring your rhubarb to a slow boil, melting the sugar.
Let the rhubarb bubble on a simmer for around 5 minutes, then test the jam on a plate, with a drop, then see if it sets. If it does not thicken, simply put it back on the heat for another few minutes until it reaches the desired consistency.
Take the jam off the boil and leave it for ten minutes before you put it into you readied containers. Make sure these are sterilised and warm before you put hot jam into them.