You need to ‘check in’ and ‘check out’ at the start and end of every journey you make by public transport. For a bus journey you do this on the bus itself. For trains, you need to use the OV-chipkaart reader at the entrance to the station. Be sure to select the correct OV-chipkaart reader:
Route between Kerkrade Centrum and Maastricht Randwyck: Arriva
Route between Roermond and Maastricht Randwyck: Arriva or NS if you use an intercity train. (also note that if you change in Maastricht from an NS intercity train to an Arriva local train in the direction of Randwyck you need to check out and check in here too).
At the start of your journey hold your OV-chipkaart against the card-reader screen, which you can identify from the OV-chipkaart logo. At the end of your journey, check out in the same way by holding your OV-chipkaart against the OV-chipkaart logo.
Also note: when you switch from one bus to another, or from bus to train and vice versa, you need to check in and check out on each occasion.