Several skeleton user supplied subroutines are available in the file user.cc. You can finish a routine, compile all files and link it to get an executable. Further explanation is provided in user.cc.
As a special option you can use an ABAQUS umat.f (ABAQUS is a trademark of Hibbitt, Karlsson & Sorensen, Inc.; see http://www.abaqus.com for the ABAQUS homepage). See the umat.c routine in the tochnog/src directory how to do that.
See also: user_umat, user_data and materi_history_variables.