Open FL Studio
Set up your project the way you want
Add channels, instruments, mixer tracks, and settings
Save the project as a new file
Go to File
Select Save as
Choose the FL Studio Templates folder
Name the file
Save it as an FL Studio project file
Open the Browser in FL Studio
Find the saved project in the Templates folder
Right-click the file
Select Add to template menu
Restart FL Studio if needed
Open File
Go to New from template
Select your custom template