I'll have a dig around, been a while since I created anything with it.
Not sure if there's any one source documenting the method. What it comes down to is some cycle-precise stuff over 3 scanlines or so, and the before/after part of looking after the display.
And, no, don't want to keep it secret. Would be so good if 480i support came to programs like RastaConverter and G2F, that would help make it more widespread to say nothing of helping to put C64 into it's (2nd) place.