First off, you need to right click the game while in your Steam library.
After that, you will need to click Properties and then Launch Options.
Then, add +com_allowconsole 1 to the text box and click Accept.
Now, launch the game, press Insert on your keyboard, write pl_FPS 1 in the console, press Enter and enjoy First Person goodness!

One thing to note is that this command doesnt count as a cheat, so even if you use it, you can earn achievements FREELY!

Thanks for reading my little guide <3
Have a good day!