Functional Specification Document needed

hi guys ...

i have been asked to write the functional specification document for the following issue... i have never wrote any functional Spec. till now... can n y one write it down for me...or tell me the exact format of how to write it down...

The issue is as follows....thx.

Invoice enhancements for tagging, serial number and email identification

1. Add the Customer Tagging Text to the invoice document.

2. Add Serial Number to the invoice.

3. Change the subject line for the emailed invoice from the generic "SPX Invoice $000000001" to the more meaningful format "SPX Invoice <invoice number>, SO <sales order number>, for <customer name>", the order acknowledgement already has a similar format

Please do reply.... thanks in advance.