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.
;If you want, add another text
titletext ["Text number two" , "plain down"]
I hope it helps