You’ll need to melt the magnesium stearate over direct heat—I’m pretty sure all the instructions in the book say so, due to the melting point being too high for a water bath.
The best way to go about it is mash it up with some liquid oil to break up the clumps before starting, and then melt it over low, direct heat; then add the wax and lower the temperature (as beeswax discolours at temperatures high enough to melt magnesium stearate—it’s purely cosmetic, but avoidable). You can definitely melt everything together all at once if you’re not fussy about the base being a bit darker 🙂
