2.1
Cut a length of bare tinned copper wire (20 AWG or similar) long enough to span from Blend to Tone pot casing — about 80mm with a few mm extra at each end.
2.2
Pre-tin the wire. Run a bead of solder along the entire length. This makes the casing joints much easier.
2.3
Pre-tin each pot casing. 380°C, hold the iron on the casing for 3-4 seconds until solder flows onto the metal. The casing is a big heat sink — be patient. Flux helps.
2.4
Tack the wire to the Blend pot casing first, then bridge across Volume to Tone. The wire should sit flush against each casing. Reheat and add solder at each joint until you get a solid, shiny fillet.
2.5
Test continuity. Set multimeter to continuity/resistance mode. Probe between all three pot casings — should read < 1Ω between any pair. If not, reheat the suspect joint.