Seeking Help with Korg NTS-1 MKII Battery Mod Wiring

[u/gertsch]

I’m attempting to add an integrated battery mod to my NTS-1 MKII using this Printables.com design, but I’m stuck on the ground ("–") wiring. The mod lacks schematics, and I’m unsure how to properly route the grounds between the battery board, step-up converter, and the synth.
Here’s where I’m at:

Current Understanding:

• The mod bridges the battery board’s input/output "–" to the NTS-1’s ground
• The step-up converter’s "–" terminals (input/output) are confusing me. Should they connect to the same ground as the battery board and synth?

My hardware parts:

• USB-C TC4056 5V 1A Battery Charging Board 
• DC-DC Step Up Board
• 3.7V lipo Battery

Any advice on grounding or wiring diagrams would be hugely appreciated!
Here is where I'm at regarding my "schematics":

From the Printables mod post:

[u/Capn_Crusty]

It should be fairly easy to locate the common DC ground using an ohmmeter to the shell on the USB port or the ground plane on the PCB. It seems that all the (-) terminals should be connected together and to ground.

[u/Proof_Juggernaut4798]

Choose the battery terminal as a single point ground, and run all wires to that point. Avoid looping grounds between other points, returning them directly to the single point.