Okay listen here buddy i know you are probably very young but don't take it personally. WE won't tell YOU the true solution to the end EVER. What is even the point in enjoying puzzle game if you can't use your own brain? Mommy won't hold your hand for your entire live so go on and try to be independent. Still don't get it you dumb little fuck? Guess what I'm gonna do it 20 times before you even finish it once hahahaha. I'm not even mad at this point, I just feel sorry for you. Even if I told you how to perform this easiest sequence in the world you'd never understand many of the strats that are so advanced that average human brain can't comprehend, so what's even the point? You'll just feel stupider at the end...
Follow instructions showed in this video: https://www.speedrun.com/portal/guide/q1frw Livesplit is setting the timing method to "automatic" by default, so make sure to change it to "engine ticks with pauses" (yes, you also need to include time from pauses in your run). Also I'd recommend to open livesplit after you launch the game, because sometimes it won't detect it, and your liveplit won't be active.
.../Steam/steamapps/common/portal (There should be hl2.exe).