Cut the watermelon rinds into chunks and make sure most of the pink flesh is off of the rind. Keep some on for color
Fill a large pot with water
Add the cut up chunks of rind
Par-boil the rind until the outside is fork tender
Drain
Pour the rind into a large heat-proof bowl
Using the same large pot, add sugar, vinegar, and cinnamon
Bring to a rolling boil
Carefully, pour the hot liquid over the watermelon rind
Place the large pot you boiled the rind in over the rind to weigh it down so it stays submerged
Leave out on the counter to pickle
For the next two days, pour the juice back into the boiling pot, bring it to a rolling boil, and repeat the process of pouring it over the rinds and weighing them down.
On the fourth day, heat the juice and rind together until boiling.
Fill canning jars with rind
Pour the juice over the hot rind
Tighten the lids
The lids should seal within a day. Test by pushing the top down. It should remain flat and not pop up.
Store in a cool, dark place for one month