Adding a custom Invoice Ref # to an order

I need to add a custom invoice number to my orders in addition to the order ID. I have already modified the order table but for the life of me I can not figure out where to place the code for inserting the invoice number. The field in the table is indexed.

Can you advise me as to where I would want this modification?

Thanks

Where do you want it to appear? on what page?

Where do you want to add the custom id ? or should it happen automatic ?

This shouldnt be to hard to realise, if you can explain my two questions i might be able to hack together some stuff which do this for you (so not a module or a plugin but just some down and dirty php code that does the job)

I have done some of this before (for a custom model etc. etc.) i have a tutorial up on my website how to do that

you can send me a pm so i can take a look when i have time for this...