You said I have managed to set my portfolio to my front page, so I assume your current homepage is using the Layout - Portfolio template and is populated by portfolio posts automatically.
So change that template to Blank - No Container | Header, Footer and then layout/design this homepage as you like in Cornerstone. Then have a section in there where you place the The Grid element or Essential Grid element (whichever extension you use)
You'll get a corresponding grid element whichever extension you use, so it is going to be easy to put your portfolio grid on the page.
It's the how to create the portfolio grid you need to work on, please refer to the documentation that I provided above.
Hope this sheds some lights,