Let’s start with the printing first. You can assemble the electronics (on the next page) while the printer runs the job.
There are two parts: a main pack into which all of the electronics fit, and a small latch piece to hold the pack in place in the iron (similar to the original battery holder). Use transparent filament so you can see the PowerBoost status LEDs inside.
The pack has some minor bridging and overhangs, but most printers should be OK without support material. If not, switch that on and give it another go.
If using a very tiny printer (such as M3D or Printrbot Play), you may need to turn the body 45 degrees to fit corner-to-corner across the bed.
“Normal” quality (0.2mm layer height), 25% infill works well. “Fine” quality (0.1mm) looks amazing but takes hours to print.
The latch piece slots into place at the back of the pack. There’s no “click” like the original battery holder, but friction should be sufficient. If needed, you can scale the Z axis to make it a little thicker or thinner, and print that piece again…it only takes a few minutes.