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 (trademark of Hibbit, Karlsonn & Sorensen, Inc.) umat.f routine. See the umat.c routine in the tn/src directory how to do that. As an example you could try our hypoplasticity implementation, which is on the Internet at:
See also: user_umat, user_data and materi_history_variables.