All you need is a very hot day (95-100F), or a barely warm oven (150F).
Chop up a pile of rhubarb and set it out in the sun (or in the oven), until dried up and shriveled. If you leave it outside, you might cover it lightly with a thin cloth to keep dust and gunk away.
Once the rhubarb dries up, pack it in sugar until needed. They get really small, so make a lot more than you think you’ll need.