How to add a custom crafting to Minecraft

Close Minecraft
Extract file 1.12.jar as a zip file
Copy the crafting json file in the folder 1.12/assets/minecraft/recipes
Delete the folder 1.12/META-INF
Compress a zip file (zip format !) with the 1.12 folder content
Rename this file to 1.12.jar
Replace the original 1.12.jar file
Launch Minecraft
And voila: your custom crafting is working in Minecraft :-)