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.).
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.