On the main page, select 'Operations'.
Any pen icon on the right side indicates that this value can be set or re-set.

Add to trend

Drag the desired trendable object to the right and long-press the blue symbol to add an input value to the trend.

The button will then change to green to indicate that the value has been assigned to a trend.
Long-press the green trend symbol again to remove the value from this trend.