The instructions below are for bulk applying a catalog to multiple galleries.
1. Navigate to the Library > Galleries area.
2. Select the galleries. The tutorial in this link will walk you through the different ways of selecting galleries.
3. Once the galleries have been selected, the bulk edit tool will appear on the bottom of the page. Choose a catalog from the dropbox and click Apply.
NOTE: If the Choose a Catalog box doesn't appear, then you probably don't have a catalog set up. The tutorial in this link will walk you through the catalog creation.