Here's the play-by-play...
1) talk about activities with her and have her agree to doing an activity with you
2) say "That's cool. Shoot me a text so I have your number" (Important... DO NOT have phone in hand while saying this or she will not take out her phone)
3) Wait. If she does not take out her phone in 20 seconds, say "Take out your phone so you can text me"
4) When she takes out her phone, take it out of her hand (gently)
5) open up the phone dial pad
6) enter your number
7) copy the number you just entered
Add yourself as a contact in her phone with your first name + a commonality you built with her (for instance, I type Chris Vegas since a lot of times, the girl has visited Vegas and that's where I spend a lot of time)
9) Open the Message app
10) Click "New Message"
11) Paste the number you copied into the recipient box
12) type out a message to the girl. The exact message is dependent on the interaction, but here is the overall idea... "Hey [your name]. It's [her name]. Can't wait to [activity you talked about]"
13) Hand phone back to her to send the message
14) Do not reply to her text while you are by her side.
15) Text her later to solidify the date
I started adding myself as a contact in her phone and noticed dramatic differences in the responses I received. Note: You will get very fast at all of these steps. Speed is very important otherwise she'll get weirded out that you took her phone and are holding onto it so long.