How do you install a new cover shade on an existing bathroom fixture?

What type of fixture? Can you post a photo?

Take the old one off and put a new one on

There should be a way to get the old one off. Sometimes you have to first remove the light bulb and then you can screw off the old shade.

Remove the bulb. Inside the shade, at the very top where it meets the post, there should be a large plastic or ceramic nut around the socket. Unscrew this counter-clockwise while hold the shade with your other hand. Once removed, the shade will easily slip over the socket. The install a new one, just reverse the instructions.

How about mod podging a new look over the existing one?