Here is another short tutorial on Windows...I'm pretty sure this tutorial works for Mac and PC, but I may be wrong.
This is how to contribute to the giant Internet library on the Internet, the Wayback Machine.
Step 1: Find Your Wanted Site
Find the site you want to archive, and go to the top bar. Drag your mouse across it until it's blue and press "Copy".
Step 2: Paste
Press "Paste" and then press "SAVE PAGE", then it will take you to a loading screen.
Step 3: Wait and Done
Wait and in a couple seconds it should save and be done. Congrats, now people can go to that page in the future!
This is kinda short, but it is pretty easy to add a page to the site.