First you need an accumulator charge kit and a bottle of N2 with a regulator. Ensure the system is empty. Check the pressure in the bladder via the charge kit. The charge kit includes a PG and valving to enable N2 to enter the bladder and to bleed excess pressure.
Hook the charge kit to the N2 bottle and fill the bladder to the desired N2 pressure. Once charged disconnect the charge kit.
If you google this topic you will see illustrated instructions. It's not difficult to do.
The reason we do this is so that when we top up the system with fluid, the bladder is compressed and increasing the operating pressure. Ensure you work out your pre-charge pressures, working volumes and safety volumes.

Usually this special tool is supplied by the mechanical seal vendor with the instruction manual.