To set a custom permalink structure for portfolio posts, go to the ‘General Portfolio’ tab in the global options (Options > Portfolio > General Portfoilo) and enter your custom slug in the Portfolio Slug field. Just make sure its not the same name as your portfolio. So, if your portfolio page is called portfolio, your slug could be portfolio-items. Once you insert the custom slug, go to the WordPress Settings > Permalinks page and select Post Name and hit Save Changes.