You may get a difference of opinion on this, so here's mine to start with.

1. Adjust the scope to the same point-of-aim as the iron sights to put you on the paper for starters.

2. Adjust the barrels to give the tightest composite group possible, regardless of aiming point.

3. Shoot a 6-shot group (3 right + 3 left) at the distance you wish the scope to be at zero.

4. Clamp the rifle in a gun-vise or similar, with the cross-hair on the aiming mark (not the point-of-impact).

5. Without moving the rifle, adjust the crosshairs onto the centre of the composite 6-shot group.

Others may suggest sighting the scope for only one barrel to optimise accuracy, but IMHO those who suggest this are stuck on bolt-guns and don't understand double rifles. The accuracy of a double rifle is its composite group, and with adjustable barrels this should be damn near as tight as a bolt-gun anyway.

Hope this helps, and welcome to the forums mate!