You have a device already flashed with Rockbox and wish to transfer music using Disc Mode (no iTunes involved).
If you need a guide to flash Rockbox to a device, I can only recommend following official guides on their website.
a. Turn off the iPod (hold the Play button).
b. Restart the iPod: press the Select (Center) button and the Menu button together, then immediately switch on the Hold slider.
The iPod should boot into the original Apple OS.
a. Switch the Hold slider back off.
b. Connect the iPod to your computer.
c. If USB mode is enabled, the iPod should appear as a storage device.
d. Transfer your files to the desired folder.
a. After your transfer(s) finish, press Select (Center) + Menu. The iPod will shut down.
b. Press any button or use the click wheel to the device on normally.
After everything is complete, restart your Rockbox database.