Chop the rhubarb then add it to a saucepan with the water.

Simmer on medium heat for about 10 minutes, until the rhubarb is soft, stir often.

Remove from heat.

Use a fine mesh strainer to strain the juice from the rhubarb. Press the rhubarb gently with a spoon to get more juice from it. You can see how I did this part in the images above. 

You will be left with juice and rhubarb pulp. You can discard the pulp or use it in baking (see tips).

Add the strained rhubarb juice to a clean saucepan with the sugar. 

Simmer for about 2 minutes, just until the sugar dissolves.

Remove from heat, cool, then refrigerate.