Step 1: Gather your things.
One medium sized piece of cardboard-strong not flimsy.
An alarm or piezo or hor or somthing that makes loud noise.
One short piece of wire.
A battery holder FOR THE CORRECT VOLATGE
A battery OF THE CORRECT VOLTAGE I'm saying this because I used a nine volt and it blew out my alarm, make sure your alarm can handle your voltage.
Siccors and electricle tape or transparent tape will work fine.
And Alluminum foil.