Cut the rhubarb lengthwise and then slice it into small pieces.
Place the rhubarb in a bowl and sprinkle the salt on top. Massage the salt into the rhubarb and then let sit, covered, for 10 minutes and massage again. It should now be quite wet and a pool of liquid (your brine) should have formed at the bottom of the bowl.
Add the nutmeg and cardamom and mix in.
Add the cinnamon stick and orange slice to your jar and then add the rhubarb mixture. Press the rhubarb down to remove any air pockets. The brine should rise to the top and cover the mixture.
Top with a quart-sized ziplock bag. I gleaned this trick from the book, Fermented Vegetables. Simply press the plastic down onto the top of the ferment and then fill the bag with water and seal. The bag will act as both a follower and weight. No need to buy any fancy equipment. Then screw on the lid (but not too tightly).
Set aside out of direct sunlight at room temperature (60-70°F is preferred) for 5 to 7 days (I let mine sit for a full week). Check daily to ensure the rhubarb remains submerged and burp every couple of days to release excess pressure. You'll notice that the rhubarb will lose its color.
Once the rhubarb is finished fermenting, remove the plastic bag, tamp down the rhubarb so it remains submerged under the brine, tighten the lid, and store it in the fridge. It will keep refrigerated for up to 2 months.