how do i reconnect battery without alarm going off?
Ask the owner for the code? Disconnect the alarm before you reconnect the battery?
You don’t, just have the remote handy or have someone else ready to turn the ignition on.
Depending on the year of your Subaru, the anti-theft system may now think that someone is trying to steal the vehicle. You can read what can happen to various modules/computers when the battery is disconnected on a Subaru here:http://www.aa1car.com/library/battery_disconnect_problems.htm