Sit the open mason jar(s) with the weed in them in a shallow pan of water and boil. Make sure to keep adding water as the level drops, and allow it to boil for about 20 minutes, give or take. Pull from the heat, make sure the lid seals are well seated, and then close them up tightly. Once they cool, you can put them away for storage. This is basically the same process used for any basic canning. The heat kills the mold spores, bacteria and other nasties, plus it produces a partial vacuum to keep the contents fresh, once the air inside cools down.
"ATTENTION: Due to limits on my patience, I can only please one person per day. Today is not your day. Tomorrow's not looking too good, either."