Does anyone know how to write a trigger that would copy a value from a custom field to standard field?

I have never coded with apex before, so sorry if my question doesn't make sense.

We have been using a custom picklist to store certain info about our clients, but custom fields are not available for a PDF merge.

Our app exchange partner has suggested that we develop a trigger to copy the value from the custom field to the currently unused

"Industry" standard field, which is available for merge.