How do we convert the seismic trace wavelet to zero phase which is in mixed phase?
My preferred way of doing this (I've been working in seismic data processing and analysis for over 20 years now) is: 


Start with theoretical (or notional) source wavelet.
Shape the source wavelet to zero phase (or minimum phase, depending on your
application).
Design a cross-equalization filter that takes the input
from step 1 as the source and step 2 as the target. This can then be
globally applied to your data.


There are other ways to do this (like with Wiener deconvolution) that make a statistical estimate of what the wavelet should look like, and this was considered acceptable in the industry for many years. However, such statistical processes are not acceptable for all applications, such as 4D (time lapse seismic) analyses, so I prefer processes that are deterministic rather than processes that rely on a stochastic assumption.