Once you have connected Zotero via browser extensions, you can search for articles on Google Scholar, and a folder icon will appear to the right of the address bar. Select that folder icon, and a box will entitled "Zotero Item Selector" will pop up. Check the box to the left of the individual titles to select the citations to save. Then select OK. The citations will be saved to your Zotero account.
Import book citations from the library catalog:
"Export RIS" will export the citation to your Zotero account. Select the ZoteroBib icon to export to ZoteroBib.
Open your Zotero app, and select the library where you want to add your citation. To manually enter a citation, select the page icon with a little plus sign on it. A drop-down box with different types of data will appear; select the corresponding option. This step is shown below in the screenshot.
Then, input the Metadata manually in the corresponding categories. An example is shown below.
You can add to your library directly from within Zotero with the ISBN, DOI, or other Bibcode by selecting the magic wand icon. Paste the DOI into the box and select "Search". If found, Zotero automatically adds the item to your library.