Tap “+ Add Meal” on any empty slot for a given day and meal type (breakfast, lunch, dinner, snack).
New meals start as Draft - they're penciled in but don't affect anything else. Switch a meal to Planned once you're actually going ahead with it, and its linked ingredients start counting against your inventory and shopping list.
Tap the eye icon in the filter row to hide every slot with nothing planned in it, so the week only shows what you've actually got scheduled. Tap it again to bring the empty slots back.