First, if you want to convert an existing corpus tab, you can quickly convert the entire tab to a library by going to tab options and clicking "Add to Library"
On the next screen, "Select All" should be selected. You have the option to choose which particular apps you want to add. On the right hand side you have the option to add all the sources from the tab to an existing library or create a new one.
Since you are adding an existing tab you will likely want to create a new library. Choosing "Create New" will take you to the screen below where you can name the library and choose the tab you want to add the Shared Libraries app to.
Choosing "Remove Original Apps" will delete all the apps that were added to the new Shared Library so there are no duplicate sources on the dashboard.
Second, you can go to the top left of your screen, click "Add+", then click "Essential Apps" on the left hand side of the screen and look or search for the "Library" app.
This will create an app on your dashboard (see below):
Click the edit button on the top right hand side of the app (seen above) and click "Edit". This will take you to the following screen where you can import an OPML file.
The third way to create a Shared Library app is to click on the "Edit" button on the top right hand side of any feed app (RSS Feeds, Search Apps, etc.) and click "Add to Library" and follow step one. You can create a new Shared Library app or add it to an existing one.