how do i use the micro sd card to save disk space now that i have it?

If you have it plugged in it should show up in Finder. This is my micro SD card.

Here you can move/copy your files to the SD card.

If your SD card doesn't show up you might have to format it first.

You'll find Disk Utility in Launchpad -> Other. A group of Apple apps. Mine was formatted as ExFAT, which works on all systems like Mac, Windows and Linux.

Hope this helps.