Here’s the schematic when using a Gemma board. Simplicity itself! The battery isn’t shown here…it simply plugs into the JST connector.
The battery charger isn’t part of the circuit. Unplug the battery from the board and use the USB charger to top it off.
Since there’s no JST battery connector on the Trinket, we’ll need to make one using the battery extension cable. Cut it a couple inches from the socket end (where the battery plugs in), separate the red and black wires about halfway, and strip about 1/4" of insulation from the ends.
The red and black wires each need to connect to two places in the circuit: red goes to BAT+ on Trinket and + on the backpack, while black goes to GND and –. These connections require a split inline splice. Even a regular 1-to-1 inline splice can be tricky for a novice, 2-way is an extra challenge. It’s a little easier with narrow-gauge wire (e.g. 26 ga.).
This lets us route battery power to both the Trinket board and the LED matrix.
Line up the 2 wires on one side and twist them together. Slide heat-shrink tube as far down as possible. Then bring in the power wire from the opposite direction and twist around the others. Solder and heat-shrink the connection.
Work slowly and methodically, remember to slide the heat-shrink tube on FIRST before joining the wires, and use proper soldering technique: heat the wires and apply the solder there; don’t move a glop of solder from the iron to the connection.