There is a guide in the Abaqus documentation. You can get that in Abaqus User Subroutines Reference Guide / User Subroutines / Abaqus/Explicit  Subroutines / VUMA.

In the element type menu in the mesh section there is an option for element deletion. There are some sub controls for fine tuning it. You can turn that on for simple procedures. Unless your problem is quite complicated it should be enough. You shouldn't need vumat . Hope that helps.