For backing up an SD card the same strategy would work, but in reverse: you create an image file from the SD card in the sd card reader.
Indeed, it’s turtles all the way down:
The only gotcha is you don’t want to make a backup of an SD card that is actually running the OS, so you need another machine to do that (or another SD card to boot your same raspberry pi) to make a backup of another SD card.
It might be easier and certainly faster from a PC, but it’s certainly doable from a RP alone, you just need an extra SD card with an OS containing DD that won’t be touched, and external SD card reader/writer. One you’re there make an image of the original SD card to an image file (using dd) and keep it somewhere safe in case ou need to restore it (again, dd will do that).
Once you have the backup, boot from that image to actually attempt to WRITE a NEW image to a new SD card (now knowing you have a backup of the OS you’re running in case you mess up).
If all is succesfull you can try and boot the new SD card.
I’ve never attempted it this way, but you’re sure to spend a lot of time on this and have some exciting moments 
Or you could simply do it from a PC… It won’t lock up the PC for very long…
Here’s some more light reading if you’re really heading this way: