If you price out individual cells and materials, you'll likely find that you'll actually be spending more money by doing it yourself. Also, please consider that some lithium batteries if handled or charged incorrectly can enter thermal runaway... AKA, cause a huge fire with extremely toxic smoke.

With that said, if you want to learn how to do it, and how to do it safely, there's no reason you can't. You will need to choose a cell/chemistry first, and figure out how many cells you need in series (+ to -) to get your desired voltage. Then you can figure out your watt*hours depending on the capacity of those cells.