Open your target spreadsheet, or the spreadsheet you want to link to.
Next open the source spreadsheet, which is the spreadsheet you want to link from.
Both have to be opened in the same "instance" of Excel.
To have both target and source spreadsheets appear side by side:
Options might need to be changed to: Enable this content.
In the target speadsheet, hit = in the cell you want to link to.
In the source spreadsheet, click in the cell you are linking from and hit <enter>.