Sealed heating systems, as are almost always found with combi boilers, need to have their pressure checked and topped up from time to time. In most cases the pressure gauge will be on the front of the boiler and should register 1.0 to 1.5 Bar.
Over time the pressure will fall. How quickly depends on how leaky the system is. Very small leaks often occur on radiator valve spindles, Automatic Air Vents (AAVs) and other components. Often you will not notice the leak since the small amount of water will evaporate.
Recharging the pressure once a year is probably normal, but once a month or more excessive. Every time fresh water is introduced, further scaling and corrosion may occur in the system, so you should not leave an excessively leaky system to continue.
To recharge the system pressure you need to locate the filling loop. This is usually a silver coloured, flexible braided connection with a small valve at each end. Usually under the boiler or nearby, but sometimes incorporated into the boiler (Vaillant Turbomax Plus and some Worcesters for example).
Having identified the filling loop, you need to work out what control valves there are. The first control valve will be nearest to the connection to the mains cold water pipe. This valve may incorporate a Double Check Valve (DCV) or be followed by it.
Then there will be the flexible connection which is usually left connected at each end (although in theory it should be disconnected - if yours is disconnected, connect by screwing swivel nut at the end). At the other end of the flexible connection (loop) will be a DCV and/or another control valve, then the connection to the return heating pipe.
The control valves may have plastic levers or screwdriver slots. In each case, they are on if the lever/slot is in-line with the valve (length ways) and off if the lever/slot is at right angles to the valve. There may also be a screwdriver slot on the DCV - do not unscrew this as water will be released.
If there are 2 control valves, practice turning each one separately (no water will flow unless both are open). To recharge, open the control valve(s). You will hear the water flowing and observe the rise in pressure on the pressure gauge. Close the valve slightly if it is filling too fast. When the gauge gets to 1.0 Bar or a little over, turn the valve(s) off.
Make sure that the valves are properly closed, preferably by disconnecting the filling loop to check that no water is flowing. Operate the boiler and check the pressure again. Any air in the boiler will vent and this may result in a drop in the pressure, requiring further topping up.
You may also need to vent air from the radiators, particularly those at the top of the system and near the boiler. When you do this system pressure will drop again, so top up again. After this the pressure should remain fairly stable, although a slight increase on heating up of system (and drop on cooling) is normal, as is a slight change when the pump operates.