First thing to identify .... If its a JDM they have a very small battery which are prone to drain, especially if its a few years old and getting punished in the cold weather.
Removing the battery is very easy.... few trim panels held in with plastic clips. Once off, two battery clamps on each terminal and then two bolts on the battery clamp.
Best thing to do it identify if the battery is at fault, if so replace the battery. (Places like halfords can perform a battery check Free of Charge)
If the battery is good... you need to source the drain - easier said than done