Rhubarb is always sharp and astringent when raw, so is almost always cooked down with some sugar to temper it. How much sugar can be up to you, but we’d suggest that the sharpness is all part of rhubarb’s charm, so you should use a modest amount; just enough to take the edge off. Rhubarb is also very easy to overcook, and can lose its shape quickly. This is fine if you just want some stewed rhubarb to dollop on your breakfast, but not if you want to keep shapely pieces for a photogenic dessert