A perfect slit?, drill a hole the height you want the slit, then draw a level line down from that. Connect the line to the hole by cutting with a grinder, handsaw, or whatever metal raping utensil you have.
If you want it incredibly perfect, you can find the middle of the steerer tube's id, then you can make a line from that, then..... You know what? I don't want to explain that. lol.