1. First of all you need to create a new Portfolio item. Go to Portfolios -> Add New, set a title and a featured image.
2. In the same edit screen of your portfolio item, in the Portfolio post type meta box, set the Portfolio Single Page style as Page Builder.
3. Add a gallery module in the page builder and upload / select your images there.
Repeat the steps to add all your portfolio items and once that is done, you just have to create a portfolio grid.