Craft green dye with cactus green:
Smelt cactus in a furnace to get green dye
Craft green dye with plants:
Put a green plant (any cactus or green plant item that yields green dye in your version) in a crafting grid with bone meal or another dye ingredient if applicable to your version
Use a crafting table or furnace depending on your Minecraft version and available recipes
Verify the recipe in-game using the recipe book or JEI/REI (if installed)