Excellent :)

Have a look at the bit on the manuals regarding this.

http://hansamanuals.com/main/english/mailnumber___58821/theconf___477/manuals/levelzero___58821/version___84/hwconvindex.htm

Its more complicated than it should be really. They key factor is whether or not you want to create another invoice from the sales order after the credit note is raised. This works nicely if you do, but not if don't want to.