Simply using storyboard, constraints to set size of cell in UICollectionView

Here's a UICollectionView, and the cell in purple:

Quite simply, I want the cells to be 1/2 of the collection view width. (So TBC, it will be a two rows arrangement of cells in the collection view.)

(The collection view is simply fullscreen, so each cell is half the screen width.)

How do you do this in storyboard?

If I try to control-drag in the normal way, it basically doesn't work.

These are simple totally static cells (not dynamic).