Ari, if you are Jewish you should know, but I will explain. You cannot write your own Mezuzah if you are not a scribe. You cannot use paper or pen. Mezuzot are written on a special parchement from a kosher animal. There are thousands of laws regarding how its written and the pen is a special one, I believe a feather as its used for writting a Torah scrool. The script is not exactly the same as while writing Hebrew in a book or a letter.
I believe that in Torah times many people knew that requirements and wrote their own Mezuzah as it is commanded "You shall write them", but I'm not sure. Currently, it's impossible to do it.