Image Grid for Main Page from Custom Post Type Posts

Hi, so I have made ( without plugins ) a custom post type for Items, added a metabox for custom fields and everything works just fine so far.

Now I do need to make a shortcut to display a grid of featured images from those custom posts on the main page, preferably with the best image size chosen for the current display size. I do have a rough idea how this works but I am lost right now. Basically I would like to use something like this [item_grid number=8 columns=4] .

Could you maybe point me in the direction of a tutorial for achieving this or give me some sample code to analyze or anything ? Thank you very much !