Click the paperclip icon to the left of the row you’re attaching a document to.
This opens an attachment panel on the right side of the screen.
Select “Attach Files to Row X” and select “Google Drive”.
You can attach OneDrive or Box documents by clicking those buttons, or Upload a File directly from your computer.
Sign into Google Drive as prompted by the popup and select the document you want to attach. Click “Select” and your document will be linked to that row in a Smartsheet document.