Thanks, @Werner! I believe I can do it once I find clear instructions. There's still a lot I don't understand—where all needed things are. I'd appreciate it if you could just provide a link that could point me in the right direction.
Meanwhile, yesterday I somehow managed to install the headers using apt-get install linux-headers-current-rockchip64 . At least, for now, this allowed me to build the packages that needed the headers.
It seems like this might be a bit of a dirty trick (no?). So, I'd still like to do it right.
Stanislav Chizhik's post in How to install correct linux-headers? was marked as the answer