Which Kohler 18.5? There should be a tag with a model, spec, and serial.
I was hoping to be able to give you exact information, but the general information is to disconnect the wire from the fuel solenoid, shut off the fuel or pinch the fuel line. Have a container under the carb to catch the fuel. Use an open end wrench on the flats of the solenoid to loosen and remove it -- it also serves as a bowl nut. Once you've removed it, the bowl will come off. You might have to wiggle it a little, since they often stick the to gasket.
Hope this is helpful!