Place something capacitive on-screen
to measure its weight.

If it is not capacitive, place a spoon, set
spoon as the tare weight, then place the
object on the spoon to measure its weight.

Touch the weight to set the current
weight as the tare weight.

If you have changed your device's 3D Touch
Sensitivity, set it in this app's settings.
Weight Unit
3D Touch Sensitivity