1) Go into the Main Zoo Tycoon Folder and go down to where all the ztd files are.
2) Open up the one that is relevant to the hack you are trying to make. (Example- if you are trying to do something with animals you would open up the animals.ztd or the animals2.ztd.) Of couse if you were doing something that involved Dino Digs or MM you would open the Xpack1 or Xpack2 folder and go from there.
3) Inside the ztd file there usually are lots of files in it. You have to decide which ones you need. Sometimes you only need one other times you need them all.
4) Extract the files you think you'll need. Never work on the original files or you will most likely will find your self reinstalling the game.
5) Open up the extracted files and change the lines that will result in making your hack work.
6) Winzip the files into a zip file. Then rename the zip file to a ztd file.
7) Place the file into the updates folder. Now you have to test your hack to see if what you changed in the files was successful or not. Play the game and check to see if you hack worked or not.
8) If your hack worked on the first time than your done. This usually never happens so if it didnt work go back to the files and change them.
9) You have to just keep changing the files and testing from now on until you have a hack that gives you the desired results.