Use the battery tabs to solder the two batteries together. The tolerances around the battery pack are tight, so keep the overlaps and solder to a minimum. You solder the negative to negative so the batteries are in parallel. Take a look at the batteries and you'll see they have an indent on the body at one end.

Solder the voltage regulator board between the batteries. Be careful not to heat the batteries too much.

I used tape on the two ends of the batteries, but a heat shrink tube would work better.