Copy whatever songs into the folder.
Enjoy your own music in game.
Right-click anywhere > New > Shortcut
Click Browse in the dialogue box that opens
Find the folder where you keep your music, be it the default or elsewhere, and select it.
Give the shortcut a name if prompted, then click finish.
Run GTA V
Go to Settings > Audio and do a complete scan. The game will notify you when it is done, so feel free to play in the meantime.