The undercarriage jig is simply a length of 3x1 timber with a centre line; various screws and panel pins are used to hold the separate pieces of piano wire in their correct positions.
Unless you are exceptionally talented at wire bending and / or very lucky the joins will need a bit of tweaking to get a good fit. When all is ready the joins are held with a twist of thin copper wire, if you don’t do this the unequal expansion as you apply the heat will almost certainly “spring” the joins.
The silver solder flux is “active”, that is it physically cleans the metal as it gets to the correct temperature, but don’t use this as an excuse not to thoroughly clean the joins before you apply the flux and after soldering make sure that any flux residue is cleaned off as it is corrosive.
The thin copper wire will also be removed, as it does nothing for the strength once the join is soldered.
Just the anchor points for the bracing wires to be added and then the assembly can be clad with wood.