Using a level (digital is better, but spirit will do), determine the initial angle of the setup.
Precision is not so important here. With the bike on a level surface, determine whether the saddle sits level, nose up, or nose down, and the degree to which it is so. Take note of how the height of the handlebars relative to the saddle.
Using a 6mm allen key, turn anticlockwise to loosen the saddle binder-bolt on the underside of the seatpost. Loosen it enough to allow the hardware to be moved around, but do not completely unthread it.
Using the level as your guide, move the saddle angle as close to level as you can get.
When in doubt, err on the side of the initial angle measurement, i.e. if saddle nose was initially higher, keep the final angle with a slight nose-up tilt as well.
If the handlebars sit high above the saddle, a nose-up saddle should work best. If the handlebars sit low relative to the saddle, a nose down saddle may be preferred.
With the angle decided, tighten the bolt by turning the 6mm allen key clockwise, ensuring that the proper angle is maintained as it is clamped.
In extreme cases, the angle of the saddle can have a noticeable effect on the height of the seatpost. In case of any adjustment, it is always a good idea to test the equipment afterwards, and to allow time for the rider to adapt to the change.
Should you need to, you can adjust the seatpost height by use of the seatpost binder (5mm bolt or quick-release).
If you have questions about this, see my guide on how to adjust seatpost height! Take care!