You should check with the RWMA - Resistance Welding Manufacturer's Association.  They are an AWS associate organization and have the most process information that might assist you.  They have a Resistance Welding Manual that might have some process settings that might assist you.  Other than that, companies that do resistance welding usually develop their own process set-up sheets.  I have set up resistance processes for automotive and aerospace applications and nearly always worked to a code or published specification.

The easiest way to develop a resistance weld process from scratch is to first set force, then set time, and then set current until the teardown evidence exhibits strong welds.  With a roller process you will have fewer concerns about electrode wear than with a spot welding process.  Contamination might cause some issues though if you encounter things like an oil film.

To optimize the welding parameters if the welds are weak you will need to reduce the force and/or increase the time and/or increase the amperage. 

For galvanized coated materials the coating must be displaced prior to forming the weld nugget.  If the heat input is too weak, only the coating will bond and the welds will break.
One way to ensure that you are forming good welds is to start welding with no spark expulsion events and then gradually increase the heat input until sparking expulsion begins to occur.  This is not the way a perfectionist might want to weld, but it offers a practical check on the welding process. 

People who run robotic automotive spot welding systems like to weld too hot because they can watch the robots to see if they are sparking.  If the sparking stops they check the weld systems for service.  This is not optimal though because copper electrodes are expensive to replace, but most plant managers consider that cost to be less expensive than payouts for lawsuits from broken automobile welds.

Hope this helps.  Sorry if it was long winded.