You are not doing something wrong, you just don’t have the experience to confidently work with the data and methods. Some people pursue a PhD in InSAR and have already worked with this technique for five years during their studies.
PSI seems the correct tool to use, so you can stick to it. Did you see these wonderful collection of tutorials and advices? StaMPS - Detailed instructions - #2 by ABraun
The best way is to select one tutorial and try to follow along. As soon as you have a question, check if it has been answered in this forum and if not, ask in a suitable topic. But make sure to as as specific as possible. “I don’t know how to do it” makes it hard for us to answer - so instead tell us what data you are using, what you already did with it and at which step you are stuck (or what about it is unclear to you).
As an alternative to StaMPS you could use PyRate, which is now also officially supported by SNAP, but requires more careful preprocessing.