Open Fortnite
Go to the Jam Stage or Music-related mode
Open the Jam Tracks menu
Select a track to start
Add other instruments or stems from available Jam Tracks
Match the tempo and key if needed
Layer drums, bass, vocals, and lead parts
Adjust track order to fit the mix
Mute or swap parts that clash
Test the full mix
Save or perform the final mix
Repeat with different track combinations