Don't.
1) I'm going to assume that you do not have permission to be sexual with her?
2) It's nearly impossible to have sex with a spayed female vaginally. Even if she was a late spay, she'll probably be too tight.
3) Spaying a female dog leaves them pretty much incapable of wanting nor enjoying sex.
4) Your advances will more than likely make her very uncomfortable and could even boarder into rape territory.
This is just a bad idea, overall.