Using wire cutters, carefully snip the JST connector from the LiPo battery at about 3 cm from where the wire joins the battery. Be sure to snip one wire at a time to avoid creating a short between ground and power wires. After removing it, save the JST connector for a bit later, when we'll use it in a USB C to JST power adapter.
Take the USB C breakout board (bottom left of image) and identify the pads for power and ground connections on either side of the small PCB. Tin both pads with small blobs of solder.
Next, snip through one of the battery wires, then strip and tin the end of the cut wire with solder.
It's best to solder the first battery wire before cutting the second to reduce risk of shorting the leads together. Lay the tinned end of the cut battery wire on top of the corresponding breakout board solder pad (red wire <-> Vcc, black wire <-> Gnd), and melt a bit of solder on the tip of the iron. Holding the wire in place, touch the tip of the iron to the wire as it rests on the pad. This will melt the solder and connect the wire to the pad.
Once both wires are soldered, heat up the hot glue gun, and squeeze a small dollop of hot glue over the solder connections to secure and insulate them.
Cut a small length of 1cm diameter shrink tube and slide it over the end of the connector so that it covers the wires and breakout PCB. Don't let the shrink tube extend more than a few mm over the connector itself, or it won't fit into the QT Py's USB connector.
Heat the shrink tube until it tightens around the glue and wires. Add an additional dollop of hot glue in the end of the shrink tube near the wires to provide additional strain relief.
Electrical tape secures the connector and provides extra protection to the battery. Position the USB connector over the battery so it is oriented perpendicular to the battery's long axis and the length of the connector (but not the breakout PCB) extends past the battery's edge.
Cut a piece of electrical tape, then firmly wrap it around the connector and battery twice lengthwise.
Cut another piece of electrical tape, and slice it in half lengthwise to form two narrower tape strips.
Wrap the narrower tape pieces around the short axis of the battery, perpendicular to the first piece of tape. Attach the strips so they sit just next to the USB C connector on either side.