That's a far more complex question as buff really is made out of several different genes, and there is more than one way to make a buff bird also. I don't know what buff orps have. That said, probably expect the crosses out of buff orp and barred to be similar to Black Stars (darkish body with brown on hackles, chest= black stars are a sex linked cross between RIR and BR hens). Pick a barred cross bird and breed back to a buff and you should hit on a few solid or nearly solid buffs with barring, and it's very straightforward from there(breed birds with barring to pure buffs each generation).
How to start doesn't matter much, however if you use a barred hen that will be a sex linked mating and you will be able to use only the sons from that.