In order to perform PSI, the same procedure as InSAR is followed in terms of processing the data (TOPS co-registration, interferogram formation, deburst). Deformation can only be measured over available PS. High density of PS can be identified on building and roads (objects that do not change rapidly in time) while very low density of PS can be identified in vegetated areas.
For selection of PS , phase stability and amplitude dispersion are used.
StaMPS is by far the most popular method for performing PSI (I have never used it myself as i do not have licence for matlab).
What you need to do is to create interferograms for your area of interest, deburst and stuck them (elevation band is also required i think). Then you can export this file into StaMPS. If you need more information on this, you can visit the discussion in this forum about staMPS and PSI . How to prepare Sentinel-1 images stack for PSI/SBAS in SNAP 5
Below, you will find a PDF where it explains what command to run for performing PSI in StaMPS.
D4P2a_LTC2015_Hooper.pdf (1.1 MB)