Disconnect the Volume pot from the circuit. Save the wire going to Vol 1 (this is a ground wire). Also make a note where Vol 2 was going on the stomp switch.

Use a 3PDT for the Select stomp switch, and wire 2 of the 3 rows as normal. Now there should be three lugs open: Let’s name them “green”, “center”, and “red” (to figure out which is red/green, look at where the tone pots connect – or just guess and be prepared to swap them later!)

Run a wire from the “center” lug of the Select stomp to “vol 3” on the board.

Run a wire from the “red” lug of Select to lug 2 of Red Volume.

Run a wire from the “green” lug to lug 2 of Green Volume.

Run a wire between Red Volume lug 1 and Green Volume lug 1.

Run a wire between Red Volume lug 3 and Green Volume lug 3.

Connect the wire you saved in step 1 (the ground wire) to either Red or Green Volume lug 1.

Connect Red or Green Volume lug 3 to the Bypass stomp switch, where Volume 2 used to go.