You're doing step 1 right now. In this step you start your grid. Every grid is born as a box without a top. Each box has:
During this step you modify and grow your box. You'll be able to add rows and columns and remove walls.
If you're creating a grid to fill a space such as a drawer, the box for a board game, or a cabinet then start with a box that fills the space (leave a small gap-- maybe Xmm/Y inches) and then use 'insert' to add rows and columns inside the box.
If you're creating a holder which will be free standing such as a shelf or tabletop holder, then start with a box of a useful size and use 'add' to grow your box.