How to mill out for over-flashing in a horizontal facade

[torrum]

Hello,

does anyone know how to mill a groove in the facade for a window flashing? It should just be a thin metal milling so you can insert the flashing 5 mm deep into the board.

Suggestions on tools and model? Something you can rent at places like Ramirent or where you can buy it?

Please explain how you have done it?

Thanks in advance!!