First you derive the displacement from the unwrapped phase, either with the module “phase to displacement” or manually entering the equation on the band maths. If the latter is the case, you need to reference it to zero displacement manually.
Please simply read the first couple of posts in this topic, everything is explained there already very well:
A tutorial is given here: Proccessing SAR images - #14 by ABraun
Please also read the notes two posts below, some parts of the tutorial are outdated and no longer required with the current version of SNAP.