Once you created a score, you can customize the brackets in the document settings. To manage the instruments and part settings, click on the instrument button on the left on the toolbar, then click on "Manage instruments".
Then click on the "Add/Edit Brackets" button at the bottom of your instruments list
You can then add and edit your existing customer brackets on this page. Click on the switches to enable/extend or disable a bracket. You also have a quick overview of the group of the left of the interface, and the preview of the score on the right part of the screen.
For example, you want to create two brackets: Flutes and Clarinets. Click on the switches between the parts to create the two brackets.