To start a digital catalog, you will first need to create an assortment with all of the products you wish to add to your digital catalog or line sheet. Please reference article here for details on creating an assortment.
One your assortment is created, click into the assortment by clicking on the Catalogs/Assortments tab in the top navigation.
Your newly created assortment can be found in the My Assortments area as it was created by you.
From there, click into your assortment. On the top left, you will see a drop down labeled My Creations. From here you can access any catalogs or line sheets already created from this assortment or you can start a new one.
You can also follow the above steps by clicking on Create a New Catalog for assortments that have not been created by you.
Please see articles in this section for further detail on creating a catalog or quick line sheet.