If you call her...
-speak up and speak clearly. Your tone is her only indicator of your confidence. No body language here.
-and you're nervous, call up to a friend beforehand. This will get you in a social mindset and dampen the nerves a bit.
If you text her...
-don't ask her to go out through text. Save that for whenever you see her next, or if you must, a phone call.
-match how long it takes for her to text you back. If it takes her 5 minutes to respond to a text, it should take you 5 minutes to respond back.
-read the last half of this --> the-rafc-stage-and-text-game-a-must-rea ... highlight=
other than that, it's pretty straightforward.
good luck,
white_knight