how do i wire my timer?

Do the two black wires go to the existing light switch? You will need to identify which black wire is the load (light fixture or switched outlet) and which black wire is the power source. This will be difficult without a meter or voltage tester. The timer black wire connects to the "power source" black wire, the timer red wire connects to the "light fixture" black wire and the white wire connects together with the other two white wires. The greeen wire needs to connect to the bare copper wires inside your wall box.

at the top of the case is a spring clip. If you push on that the inner workings pop out as a unit and you will see how the contacts work, it will be more obvious how to hook it up. The wiring diagram they give is lousy. If you water heater is 120 volt and you have a 120 volt line comming in, I would move the two thin white wires. The one on A terminal, I would move to 3 terminal the other wire should be on 1 terminal. The power wires should be: white to 3 terminal, black to 1 terminal. water heater should be: black to 2 and white to four.. If your waterheater is 240 volt the lines from the wall and heater should be installed as above. BUT the little white wires on the terminals that run the timer must be removed and hooked up to constant 120 volt power suplly