As far as I understand, the most important thing is tinning the barrels and ribs separately, using a strong flux, and then clean VERY well the flux rest so there is no corrosion in the future. Once all places together with the wire, apply the rosin flux, and parts should solder nicely as they have been previously tinned.
I am concerned with the "rosin flux", as I am not sure about what the correct translation into Spanish and I don't know if I will be able to find it locally.
Rosin flux should be available in Spain. It is a natural resin extracted from pine tree stumps or bark, known as rosin or colophony. I Googled the Spanish translation for rosin, and it is "la colophonia". It consists largely of abietic acid, which acts as a cleaning agent when heated to remove oxidation. It is very commonly used as a flux for soldering electrical circuits, and a lot of electrical solder has a rosin flux core.
The 97% tin/ 3% copper solder you bought is intended as a lead free substitute for copper water lines and fittings. It won't work well for relaying a shotgun rib. The Oatey No. 95 Lead Free Tinning Flux shown on the bench in one of your YouTube videos is great for tinning, as it is infused with tin particles. It works very well with regular tin/lead solders. It is corrosive, and any residue must be cleaned and neutralized.
I'd strongly advise that you find a set of junk double barrels to use for practice before moving on to your Scott barrels. You can learn just how much heat you will need to unsolder the ribs, and you can then learn the entire process of cleaning, tinning, and soldering, while also finding an effective way of securing everything in position while doing the actual soldering. Just jumping in with good intentions is likely to end up badly.
