My advice is to not tell her at all. What would that accomplish? She's likely to be hurt and/or angry. Do the both of you a favor and don't tell her.
There isn't a proper way. <br />
You should have thought of that before you dropped your pants.
rent a billboard that they will pass on their way to work or home
Maybe you'll get lucky and she'll tell you she cheated too....that way she can't get mad at you !!!
She's gonna be furious and extremely hurt no matter what.
Lol she will be mad either way. Don't try to sugar coat to lessen the pain. Tell her, in person, that you cheated on her. After she curses you out, walks away or beats your face in, apologize explain but never make yourself look like the victim. That's just fvcked up.