Well first you have to have the files for the music saved onto the SD card. Either ones you have downloaded or ones you copied over from another device.
Next the music files must be in a format that your tablet will recognize and allow to play.
Then you have to have an app that will play the files. Either the music app that came with the device (often a Google music app) or install another app that will play the files.
Now you just need to open that app and have it search for files. It should allow for searching both the device and the card.