How do you put music on a Lexar memory disc?

To connect the SD memory card to the computer you need a USB memory card reader. Plug the SD memory card into the USB card reader and then plug the card reader into the computer's USB port.
The computer will see this as an external memory card, you can then click on this memory card (it maybe the D or E or F drive) and select the music files from your computer then copy and paste your music files to this memory card. I presume the music files you need to copy are MP3 or MP4 files.