User subroutines are a separate piece of fortran coding which can be added to the end of an input deck or added to the command line when you run the job. You don't add it in CAE though you refer to it in the particular line of input data by adding ,user
With all due respect, I doubt that it is in any way possible to explain to you - within the scope of this forum - how to develop a UMAT subroutine, unless of course you have a pre-written UMAT that you just need to add in. And even in this case, the best - and probably only - place to look for reference, including how to merge in the UMAT, is the ABAQUS reference manual.