There are several ways to add data to Library. The basic one is adding entries one by one. Entry is a structured entity of data. Its structure depends on the fields that have been added while the Library was created. That is why creating an entry is a unique process strictly determined by Library properties.
To create a new entry, select the Library and click Create entry option from Edit menu (1), press 
icon from the toolbar (2) or choose the relevant option from the contextual menu of any previously added entry (3). Also, you can use the hotkey — Ctrl + N:
The new window will be opened. Here, you will need to add data to the predefined fields. Some fields may content a hint that describes what should be added here. Some fields may be required (marked with a red star) while others are not. Also, some fields may be calculated, so you don’t need to enter anything and their data will be calculated from other fields:
After you added all needed info, click Save to add a new entry or Cancel to discard the changes. Anytime later you will be able to edit entered data or remove the added entry.