Salesforce Classic: Notes & Attachments

Uploading Notes into the traditional Notes object is pretty straightforward. Notes uploaded using this method are available in the 'Notes & Attachments' related list. Remember, these notes are classic and they lack all the advanced features of Enhanced Notes. If you are uploading notes for the first time, it is best to use enhanced Notes.

•  Prepare a CSV file with the columns ( Body : Text, ParentId: Related Record Id, Title: Text)
  
• Use Workbench / Apex Data Loader
• Object: Note
• Map the three columns accordingly and insert. 

Note that Body can only contain text and not rich text / HTML. If you have rich text, use advanced notes. Else, convert rich text to normal text using this tool and then insert

Salesforce Lightning: Enhanced Notes

There are two ways to uploading enhanced notes. 

Follow this method if you have individual files (.txt) for each note. https://help.salesforce.com/articleView?id=000339316&type=1&mode=1

Follow the method below if you only have a CSV file with the note content in rich text (HTML) or plain text format. This method does NOT work with Apex Data Loader. Use workbench or use dataloader.io.

Uploading Notes:

• Prepare CSV file with the columns ( Content: Rich Text / HTML / Plain text,
  Title: Text)
• Use Workbench
• Object: ContentNote 

Linking Notes to Records:

• Prepare CSV file with the columns ( ContentDocumentId: ID obtained after uploading notes above, LinkedEntityId: ID of the record to link to)
• Use Workbench
• Object: ContentDocumentLink