[Q] How to fully format phone to stock when I only have custom ROM installed?

As per title. I'm currently running JellyBam (excellent ROM btw) and I might consider stock and reinstall the ROM just so everything is clean and tidy again.

I have a fair knowledge of installing and rooting, so I can reinstall without problem, plus XDA is always on hand. But if, for example, I wish to do a completely fresh stock install, how would be the safest way to go about this?