Get an exhaust gasket and 2 bolts and 2 nuts and some sand paper take the two nuts off if it snaps oh well that what the replacements are for if you need em sand any rust down on the mating surface.

Place the new gasket between the 2 flanges install bolts and nuts tighten then evenly done easy job once you have the parts takes maybe 10 min depending on how bad the rust is.