The barrels are jigged so a line tangent to the tops and bottoms of the barrels are parallel. As long as the barrels haven't been cut the muzzles are clamped and any spacers between the barrels are replaced where they were originally. The tinned top rib is placed onto the barrels as they were originally and wired with wedges to hold it in place. then the barrels are flipped over and the bottom rib is put in place. If the fore end hanger was removed it is important for the keel rib between the breech end and the fore end hanger is critical so the hanger is in the right place to get the correct bearing tension between the fore end and the action. Then the rest of the bottom rib is slid under the wires and wedged. The wedges on the top and bottom go the opposite direction to help maintain even pressure on the top and bottom. Some people put rosin un before and some after but the rosin will act as flux between the tinned barrel and ribs. With everything tight heat is applied. If using a torch, propane is hot enough. Start at the breech end, evenly hating both barrels. Wire solder is pounded flat to easily fit in the joint between the rib and barrels. It is important to apply the heat mainly to the barrels so the heat moves to the rib and not the other way. Add solder to any gaps you may see. As the solder starts to flow move toward the muzzle. It will go faster as the mettal is thinner so you have to be careful not to overheat. Gravity will flow the solder down to the bottom ribs and hanger. When everything is cool double check you don't see any pinholes Then cut all your wires and clean off any solder that is outside the joint. If you didn't have the triangle shape wedges hat filled the gaps at the muzzle put a ball of steel wool in the holes and use a solder iron to fill the void with solder. I've probably forgot something, but this is, to the best of my memory, how Mr. Dennis Potter taught me to do this. Now refinish the barrels and you are good to go.
