Adding Content to Specific Page in Developer mode

Site URL: https://jaguar-mango-syyy.squarespace.com/config/

Hi,

I would like to know how can I change a specific page with in a template? I'm currently working on trying to add a grid to the homepage under the image slider but when I put anything below it, it becomes part of the universal footer.  So, the grid is on every page instead of just the homepage. I've tried to make a static page instead but I can not for the life of me, get the image slider to be full-width and I've tried dozens of solutions and nothing have quite worked. If someone had any experience in fixing or doing either one these problems, any clue to the right direction would be appreciated. 

You can use CSS to show grid on only Homepage

Where is grid, can you take screenshot?

I've attached a screenshot of the area I would like the 3 column grid (It would be placed between the slide show and the footer) . I would like to know where in the code I can place the grid. Im using squarespace developer, so I need to know which file if there is one, to place the code.