Each battery is wired to the next like this.

First, tin the ends of the wires by applying a small amount of solder.

Then hold the wires that you are joining next to each other while applying heat to melt the solder and permanently join the wires.

Positive goes to negative, leaving a red wire at one end of the chain and black at the other. This is wiring the cells in series, which increases the voltage. So here we have 3 3.7 Volt cells wired in series, giving us a nominal 11.1 V battery pack.