you should have a filling loop(a flexible hose with a tap on it) that connects from the mains cold water supply to the central heating flow, all you should have to do is open the tap, you should hear the water entering the central heating circuit, until the pressure gauge reads just over 1 bar, turn off tap bleed air from radiators, check gauge and if still low repeat until all air has gone from rads, once thats done just open the tap once again to ensure the guage reads 1 bar. ( the low pressure could be from a leak somewhere in your heating system, its worth making sure that this isn't the case ,it will save you topping up on a frequent basis)
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