ABAQUS 6.11.1 and suitable version of FORTRAN for writing a VUMAT subroutine?

I need to write a Vumat subroutine in ABAQUS 6.11.1, and I dont know where should I begin from exactly? can anybody help me with the suitable version of Fortran for this version of ABAQUS and the steps I need to take for writing a SUBROUTINE in ABAQUS?

 Thank you guys