Heres the best "advise" you'll ever get:
1.) Learn how to spell advice
2.) Take the car only if you have somewhere important to go to
3.) Take it only if you're sure you won't get caught
4.) Take note of where the seat is positioned and all the mirrors, chances are she'll notice if there is something only a little bit off
5.) Don't get nervous if you spot a cop, just act like you're doing something you're supposed to be doing (the first time I stole my mom car, I got pulled over and I could feel my heart beat in my throat.)
6.) Make sure none of your neighbors see you driving the car, or anyone else who might know your mom
7.) Be confident, and don't doubt your driving abilities
8.) Don't wreck the damn thing
9.) Park it exactly where it was and same angle and all that stuff
10.) re-adjust seat/mirrors back to original spot
Oh, and you might want to roll the car out in neutral so she doesn't hear the engine... Otherwise, goodluck.