In The Sims 4, weed is not a base-game feature
If you mean a modded cannabis plant, follow that mod’s specific curing method
Check the mod’s documentation or item interaction menu
Commonly, curing is done by placing harvested buds into a curing jar, box, or storage item from the mod
Wait the required in-game time if the mod uses a curing timer
Use the cured product from the same mod after the process completes
If you meant a different plant item, tell me the mod name and I can give the exact steps