Biggest part of it is to make sure the plasma has a good connection at the work clamp. So you may have to scratch through the paint to get good contact.

For all pilot arc machines, the rest is easy as it will do the work for you.

For non pilot arc machines, such as the SuperCut series, generally aim the arc forward slightly to direct the heat in front of the cut. Doing so will help to scour the surface to clean the paint. Otherwise, it is sorta difficult at times.

Use the minimum amperage of 20 amps to make cut. If the surface is heavily painted then a standoff will be required as the paint will melt and curl under the cutting nozzle and make it want to stick to the metal.

Move fast.