LiveSplit
LiveSplit is a popular timer used for speedrunning. It is highly customizable, and can be used for many different games. It is recommended that you use LiveSplit for Portal speedruns, because it has a built-in plugin for recording game time. This is important, because we don't time Portal runs by real time, we do it with engine ticks.
Setup
You don't need to manually create the splits file, there is a preset file at the bottom of this page.
To get started with LiveSplit, you'll need to download it from the link above. Once you've downloaded it, open it up. You'll be greeted with a small window with "0.00" on it.
Right Click, then go to "Edit Splits..." at the top. From there, configure "Game Name" to be "Portal", and "Run Category" to be whichever category you'd like to run.
Finally, you need to add all the segments. To do this, name the first segment "00/01", then click "Insert Below". Add all the segments up to e02.
The maps in order are: 00/01 02/03 04/05 06/07 08 09 10 11/12 13 14 15 16 17 18 19 e00 e01 e02
Sometimes, LiveSplit will revert the timing method to "Real Time" instead of "Game Time". If you're having timing issues where timing continues through loads or other problems, make sure to right-click LiveSplit, go to "Compare Against", and make sure "Game Time" is selected.
Preset Splits File
If you don't want to manually add all the segments, you can download a preset splits file here to save yourself the trouble. Then, go into LiveSplit, right click, hover "Open Splits", and click "From File...". Then select the file you downloaded.