Use whatever SD card adapter you have to connect your SD card to your computer.
Navigate to Applications > Utilities > Disk Utility.
Select your SD card in the sidebar. Then, select
File > New Image > Image from 'boot', where
boot is the name of your SD card.
You can keep the default settings in the Save window. I recommend selecting the compressed option to save space on your hard drive.
Select a backup location and give your image backup a name.
Don't forget to safely eject your card before removing it from your computer to prevent data corruption. You're all done!