egr vaule problem

I have a 94 firebird 3.4l and its ideling rough and is geting around 12 mpg the machanic said its the egr. . I heard about blocking off the egr so i took a pice out sheet metal and outlined the egr gasket and put bolth on the vaule and bolted the egr mount back on top. turns out it didnt stop the rough idel and made my car sound like crap when i gave it gas. so im wondering if i did something wrong?

and made my car sound like crap when i gave it gas

my english is not so perfect, hope you understand me:

have you maked the new gasket leak-proof?

btw: rough idle..... check also the plugs and the wires!!!