Go to the "Pack" tab on the gfsEdit section of SkullMod. Then, click "Select Directory" under "Pack .gfs", which should open another window, of your computer's libraries. Go "Computer", "Local Disk", "Program Files (x86)" (Not sure if it'll be the same folder for you), "Steam", "SteamApps", "Common", "Skullgirls Beta" (Or just "Skullgirls" depending on which version you wanted to mod), "Data01", then you should see the open folder, you just edited to replace the songs. Click the folder until you see "Temp", click "Temp" and press "Open". Do NOT go farther than Temp. Once you pressed Open. Put "Output Filename" to EXACTLY "music-win.gfs". Then under that, tick the "Include Directory Name" box. Click "Pack" at the bottom, and it should make a packed .gfs file IN the open folder in the Data01 folder. Check the open folder, you should see a .gfs file, COPY AND PASTE it. Go back to where there are the rest of the .gfs files. Delete the current music-win.gfs file there, and paste the new one in its place. And you're done! Just open Skullgirls and try it out!