I can answer that question, but if your plan is to go this route, I would suggest doing as much research as possible.
As far as a series circuit, it's name tells you what it means, they are in series, aka in sequence, on the same circuit. Positive to negative.
This website has a good basic rundown of how to connect batteries in different ways.
How To Connect Batteries In Series and Parallel
Learn about battery configurations in diagrams - series, parallel, series/parallel - and how to connect two or more batteries properly for increased power.
You don't even need to use those particular batteries. I just linked them as an example. One key factor is to keep in mind that not all batteries are the same, especially with lithium cells like those. Discharge rate especially as if it is below the amps being pulled it can heat up the cells dangerously.
I have often contemplated building a custom battery pack to accept cells that I can remove if needed and swap out for fresh ones kept on a charger.
