Simple: Call the next day. This is an important call, and should not be awkward for her.Here are some things to keep in mind:
- Don't call to "check in" with her.
- Don't refer to having had sex with her.
- Call because something funny happened that you want to tell her about or for any other reason why you might have called her even if you hadn't had sex the previous day.
- Be the same natural, fun, and interesting person you were before you slept with her.
- Don't let awkward silences develop, or sound nervous or talk too much or too quickly.
By calling her the next day, but not making a big deal of it, you will help to set the frame that it wasn't just a one night stand, without putting pressure on the situation (which is a bad thing).