Start off with copying or creating an appropriate symbol
in Cadence.
Copy the opamp/symbol to opamp/veriloga in UNIX and
refresh the file manager.
Open the veriloga view, there should also spawn a text
editor window with an empty verilog.va file opened.
Paste in your opamp veriloga code, true up the pin order
and naming etc. and deal with whatever passed-in variables
have to come from instance properties (these have to be
put in the symbol and veriloga views).
There are many repositories of veriloga "chunks", I like
designersguide.org
You may want to go in reverse, get the veriloga code
and hand-build the symbol and properties list from that.
As opposed to making a symbol that looks good by
itself, but then needs re-editing to match the code.