Adding a scope to your air rifle can improve your accuracy in a hurry. However, a scope won't do you any good if it isn't properly zeroed in. It is important to zero your scope when adding it to your rifle and periodically ensure that it is still properly aligned.
Draw two small dots, about 1cm in diameter, on the cardboard. One dot should be about 7cm directly below the other.
Set up your cardboard in a firing-safe location.
Lie prone about 5m from the cardboard and aim at the top dot.
Adjust the windage knob, on the side of the scope, until your shots are in line with the two dots vertically. Don't worry that your shots are not near the top dot in vertical height. They should be low and near the bottom dot.
Move to about 25m and again lie prone.
Fire shots at the top dot and adjust the elevation knob and windage knob until shots are consistently hitting the top dot.
Adjust based on distance when using your rifle in the field. It is now zeroed to 25m, an ideal distance. Shots at a shorter distance, or more than 30 to 35m, will hit lower than zeroed.