If your AirTag is new, remove the wrap around the product and pull out the tab to activate the battery. Your AirTag will play a sound and will be ready to work. Now your AirTag is on!
If your AirTag is not working, you have to replace the battery. First, remove the battery cover. You can do this by twisting counterclockwise on the back of the AirTag (where the Apple logo is) and then simply take the old battery out and replace by a new one. Your AirTag will play a sound automatically. Now your AirTag is back on!