If you do not have a silk screen pattern.
Here is a description of applying thermal compound.
Put a small amount on the heat sink where your LED board will go. Start out small so you aren’t wasting the grease, apply more if you need it in the spreading process.
Spread a thin layer of the grease all over the surface that will be covered by the LED board.
Once you have a coating of grease over the surface, place your LEDs on, pressing lightly, and let sit.
Tools to spread the thermal compound.
or you can use a plastic card like an old discarded credit card.
Here is an article that we posted about thermal management for LEDs.
https://www.digikey.com/en/articles/effective-thermal-management-of-led-arrays