Witch bottles can be made of glass, pottery or iron. To protect from witchcraft they should be buried close to the front door.
To discover a witch they are placed on a fire.
The bottle represents the witches bladder.
Put bent nails, pins, thorns or any sharp objects in the bottle and add your clients urine. Hair and/or nail clippings can also be added.
Burying the bottle in the garden will cause pain to anyone attempting to place a curse.
If your client is already bewitched then place the bottle on the fire, this will cause the witch intense pain until the spell is lifted.
There are reports of witches dying if the bottle explodes on the fire
[From my Cunning Folk grimoire]