They remove the inner structure from teh item to be done, like truck hood whatever, and then it is put into a press, they have different styles of bits I think they call them that are set up for peaked or flat style louvers, and they are pressed one at a time, if you have a guy that really knows what he is doing he can do the freehand but most of the time it is a very time consuming process and they will measure almost every louver and mark it. I just had the hood done for my 49 mercury, had 356 holes punched cost was $1000. Jim