How To Make A Girl Horny While Texting?

You have "X" girls and "Y" girls:
X girls are the wild and crazy and/or down to earth girls that know how to have fun. Y girls are the sweet and/or innocent girls that don't tend to use profanity.

For X girls it's typically about dirty talk, you would say something like:
"I would lick your clit until your vagina is so wet the head of my penis could swim as I tease you with just the tip"
For Y girls it would be more like:
"If could just kiss your neck softly working my way up to your ear has you feel the air from breathing get heavier"

In either case it's all part of the foreplay. Understand that every girl is different and it is possible that the girl likes sexting somewhere between X and Y. If you need a road map, you could always ask her friend what she likes. I don't mention girl Z because I assume sluts are horny all time but if you need a guide for that it would probably something like, "I want to bend you over the counter and F*** you hard all night long"
Hope this helps

Cheers,
Big Keegs