Once you've tinned the ribs and barrels (BTW, I tinned all of the area between the ribs and the entire undersides of the ribs for corrosion protection), they need to be thoroughly cleaned of the acid flux residue. I used lacquer thinner and a new small stainless steel brush and some ScotchBrite along with some water and baking soda. Use compressed air between the short rib and forend lug (I didn't remove these during the stripping process since they were tight.
Then the tinned joint areas need to be fluxed with rosin flux. I used a common paste type. Put it on both surfaces.
Here's where you need to insert the barrel spacers. They should be previously tinned and coated with rosin flux. Mark on the outside area of the barrel where they are located. This is where you want to place the wire to draw the ribs down. The spacers serve to prevent you from bowing in the barrels.
This is a picture of the 20/28g conversion I did a few yrs ago. The spacers are longer than most and more numerous because I was doing some regulating without the ribs attached due to the severe convergence from a larger dia breach than most 28 ga guns have. Anyway there's the basic concept of spacers.
Then wire on the top rib only, and only right near the breach end where it comes together with the stub rib that is part of the breach lug assembly. Use bars of steel bar or aluminum for wiring the rib to distribute/push the load down on the rib. One bar should bare against the short underrib. Also, wire the forend lug if you hadn't removed it during stripping, just to make sure it doesn't fall off in this process. With just the back of the rib tied down, line up the full rib with the stub rib side to side, then push it hard against the stub rib. You should have tinned this joint as well. There'll likely be some solder that won't let the two ribs come together tight. Don't fret. Heat the area until the solder starts to flow in the stub rib/full rib area and push the rib back with a stick or other handy object while the solder is fluid in the aft joint. It should go tight. Let the assembly cool.
