The code is "titletext" for example:
We say the script has as name "subtitles", so let's exec it. In soldier's init or some trigger, put this code
[] exec "subtitles.sqs"
Now in the script, you must add the following command or code.
titletext ["Text number one." , "plain down"]
;Add delay, If you want.
~2
;If you want, add another text
titletext ["Text number two" , "plain down"]
I hope it helps