There are a number of approaches you can take.
Firstly, if you read the source code, specifically the code for pw_export.x,
you can give yourself a pretty decent idea of how the wavefunctions are
stored and accessed by pw.x.
Secondly, if you use pw_export.x then you can export the wavefunction
Dear Quantum Espresso Users,
I am interested in calculating certain electronic properties of materials after
a DFT calculation. To do that, I need to be able to access the electronic
wavefunctions and perform the post-processing in a different environment.
However, it seems to me that the
