It's all in the FAQ. Undo a drain cock on the pipework, with a hose attached. DO NOT use the PRV to let water out, because you'll probably make trouble for yourself with it leaking. You won't be at the boiler, the gauge isn't particularly accurate so you won't know when the pressure is really at zero. Also the drain cock will probably be dribbling from the part you turn so you'll need to be there to watch it.
If you leave the drain cock open, air will be entering via the automatic air vent so the water will keep running out.
Which is why I said let some more water out after the gauge reads zero, then shut the valve. If you leave the drain cock open while you're at the boiler pumping up the pressure vessel, it'll lose water all the time - far more than 10 litres. On some, but only some, boilers, you could close the automatic air vent, (more likely to confuse the diyer, who has to find it and realise whether it's working or not) which will be letting air in. On many the cap will be missing, or seized, or not fitted by the manufacturer.
Then you pump up the pressure vessel, and check the system pressure hasn't risen. You can go back to the drain cock to see if any water spurts out. Not critical if you end up with some air in the system, at worst you'll have to bleed a rad. Most of it would leave via the auto air vent.
