Same way as a delivered record (or you can add audits to a delivered record if it doesn't already have them). You can either turn audit on for individual fields or build a custom audit record.

To audit field changes, double-click on the record field and choose the actions that should trigger an audit record to be written. Results are in PSAUDIT for that record name with one row per field audited. The rows are identified by the name of the record being audited and the complete key structure for that record corresponding to the row that was modified.

To create an audit record, do a save as on your custom record to another record name with AUDIT_ as a prefix. Don't copy PeopleCode. Insert AUDIT_OPRID, AUDIT_STAMP, and AUDIT_ACTN as the first three fields. Remove any fields you don't care about. Save the record and build it on the database. Then go back to the original custom record, open its properties, click to the Use tab, and assign your new AUDIT_ record as the audit record.

I would add to this that you should also update the audit delete SQR so your audit record's contents are periodically purged. If you don't clear the audit
record the potential for performance impacts will begin to show.