When should I use the overlay sheet that comes with the DDJ-GRV6?

When using the Stems FX function in Serato DJ Pro, put the overlay sheet on the Groove Circuit section on the unit.
Refer to the Instruction Manual for details of the Stems FX function.

For details, refer to [Serato DJ Pro] > [Using Stems FX] in the Instruction Manual.
You can read the Instruction Manual [here].