How to create and show portfolio on the page?

Please find below for the summary of how to add portfolio into the page:

1. Go to "Portfolio > Add New Portfolio Item" to add each portfolio item. Repeat this until you get all the items you want.

2. Go to "Page > Add New" to add a page for listing all those created portfolio items. 

3. In the page edit screen, click "Visual Composer" button to enable Visual Composer editor then click "Add element > Portfolio" to add the Portfolio element into the page. You can set some element options here, for examples, which portfolio categories to display, how many items, etc.

Note: I would also recommend to see the documentation for how to import the XML dummy data to get most posts/pages like showing on the demo and you can use them as examples. You can start from there or you might get some idea when browsing through the sample pages.