Plug in charger in laptop before plugging in mains outlet
If you connect the power supply to the laptop last, the output capacitors of the power supply will discharge to charge the  input capacitors of the laptop, causing a large inrush current spike. However, no sensibly designed laptop will ever get damaged by this.  
If you plug in the power supply last, the inrush current will be much less extreme as you will not be shorting capacitors together. However, the output voltage of a badly designed power supply might overhoot when it gets first plugged in, subjecting the now connected laptop to a voltage transient above the allowed input voltage range.  


In practice it doesn't matter at all. By the way, laptop "chargers" are technically not chargers at all. The circuitry responsible for charging the battery is on the laptop motherboard and battery itself, while the power supply just outputs a constant voltage for the laptop to use.