It should be fairly straightforward. Make sure you're licenced for the extra running hardware.

Brent Ozar has a screenshotted install

http://www.sysads.co.uk/2012/02/add-additonal-sql-instance-to-existing-sql-server-2008-r2-cluster/

Also, you need to install the 2nd instance from the 1st node, then fail it over. Make sure your SQL max memory settings are suitable (either change them through a SQL agent job on SQL startup or ensure both instance will fit in RAM on the same node)