To create a new brief, simply click on the plus button on the top right of the homepage:
A popup window will then appear to let you fill in the name of your brief and choose a color:
Then click on "Create" on the upper right corner.
You will now see your new brief on the homepage in card mode as well as in the brief list on the left.
If you change your mind and don't want to create a brief, you can simply click "Cancel" or click outside the pop-up window.