Create a new cart with a grid layout

Go to Content > Cart Layout.
Select CREATE CART LAYOUT FOR TRIGGERED MESSAGE FROM TEMPLATE.
Enter the Layout Name.
For Pre-Defined Template, select Cart layout with a variable size grid of products.
Select SAVE.

Edit an existing cart with a grid layout

Go to Content > Cart Layout.
Hover over an existing cart layout and select EDIT.
Select or choose a pre-defined template.
For Pre-Defined Template, select Cart layout with a variable size grid of products.
Select SAVE.

You can also directly edit the cart layout code to create a layout with a grid/with multiple rows. Learn how in Create a layout with multiple rows/columns.