Remove the chain from the rear derailleur cage and wrap it around the biggest front and rear sprockets. Then see where you'd have to join it, and add two links to it.

This is typically how Shimano and others tell you to do it in their installation manuals.

If the old chain did work before with no issues, then as [MENTION=35181]Reynolds[/MENTION] said, you don't need to change anything. Just use the same length the old was.