Users with permissions to handle tournaments on the site will have four options available. The one we're looking for is Seeding.
Click that link and you'll be brought to the area you need to be.
There are two ways you can go about seeding a tournament. The first is to randomly assign the teams that users have created. To do this, click the Randomize Seeding button at the top of the site.
Alternatively, you can manually set the teams for the tournament from the dropdown menus on that page.
Once it looks how you'd like, click Save and you'll be set!