There's no slang substitute without looking stupid. You could write 'wtchng' but I do not recommend unless you are on a pay-by-character plan.
gtk
Text You Back
make love to me
killing myself laughing
First, look up some slang, if you don't already know any. Most people know lots of slang. Second, imagine two people talking. Third, write down what you think they would say in slang.
welcome back (to a text or chatroom) or write back lyk tb text back
Miss You!
gtk
. On My Way
penis
lpl
IZZY
Text You Back
make love to me
Thanks In Advance
Did not laugh/does not like
you take care :)