The battery on diesel W123 cars is located in the far rear left corner of the engine bay, as you face it from the front of the car.

This battery is in fairly good shape, but has not been maintained in several years and has gotten dirty and could use some love...

To begin, disconnect the positive and negative battery terminals. This will be covered further in Step 3.

Be aware that batteries contain corrosive materials, release potentially explosive gasses, and can output a painful and potentially dangerous 12v shock. Use common sense, keep away from open flames, don't use strong chemicals around the battery, wear gloves and safety glasses etc.