It's unwise to use the battery's internal resistance to warm it up if it's already cold, as the reason you don't want them cold is that they degrade in annoying ways if you discharge them when cold, so then discharging them to warm up is.... defeating the purpose. You can use a pulsed current drain to keep them warm once they are, or even rely on the static current draw of the system to keep them warm, but that does involve good knowledge of your cells and system and some extra maths or very good guessing.
Any "normal people wire" has resistance. I have to add the quoted bit these days to avoid people moaning about super conductors, but if you're flying at such low temperatures, hire experts to help you ;-).
In a professional project I'd use a well controlled heating element to generate the heat, but in personal hobby projects it has happened that I just took very thin wire, such as any AWG38 enamelled copper I found somewhere, and wound that around each cell. Until I got to tens of Ohms total, maybe hundreds, depending on the energy I expected to need, bits in series or parallel, all is possible. You can then power that with a small amount of energy to warm up the cells from the outside, a much lower amount of current than you'd need to pulse through the battery itself, also reducing your losses in the control of that heating scheme. But instead of thin wire you can use resistive wire, that depends on what's the easier process for you. Lots of thin, or several rotations of resistive. When you use lots of thin wire, do try to use something like enamelled wire, else avoiding shorts is difficult, because you don't want plastic, you want good contact. With resistive, be sure you don't turn up the power too high, as the plastic on the batteries could melt where there's bad contact and that could cause shorts with the metal casing. Or add thermal paste and heat-shrink around the finished object.
The next step is either controlling the element with an Op-Amp circuit or microcontroller, of which many can be found all over the internet. I'd suspect "low voltage heater control" might be a good Google term.
Or making sure the leakage of heat to the outside and heat generated constantly is such that the system naturally balances between 10 degrees and 30 degrees depending on outside temperature, where you just switch on the heater in cold weather.
In either case, if you want low energy usage it will include run of the mill insulation. The better the insulation, the lower the amount of energy you need to keep them hot. With infinite insulation you need no energy to keep them warm. But that'll be ... difficult.
I would say with (very) good insulation, assuming you are aiming for good operation at nothing below -20°C outside, you should be able to do with 1W or less to keep warm. Maybe you'd need an extra element to initially warm them up with a bit more power. But again, warming up is best done with a small current patiently, as when cold, larger currents aren't the best thing for the Li-Ion batteries.
For power requirements and all such, you'll need to do some research into the lower levels of what's called "thermodynamics". Insulating materials have a thermal resistance, which means at a certain temperature difference they leak a certain amount of energy. The more layers you use, the more resistances in series, so the less leakage. Look up Thermal conductivity and/or thermal resistivity, to find what wire resistance (electrical in this case) you need search for things like "Ohm's law" and "current, voltage and power". Power in Watts relates to Joules through time, by E = P * t, I'll give you that as a gift. Where E is energy in Joules, P is power in Watts and t is time in seconds.
Once you've learned a little about the above and found out what kinds of insulation you could employ and what kinds of wire (or internal battery resistance tricks, that's up to you!), you can always come back here or to Physics or such Stacks with new questions.