This achievement is very simple. Basically, just run up to a cow and punch it a few times. It will attempt to run away but they are not as fast as you so chase it and make sure it dies. Hopefully the cow will drop some leather, pick up the leather and the achievement will be yours. (Using a sword will kill the cow quicker)
Note: Cows do not always drop leather, so if it doesn't drop any; then just look for another cow to kill, they usually come in groups anyway.
If you need further help, or would like to see it with your own eyes before trying then just watch the Youtube clip below, credit to PaLoWzZiE for uploading the video.
Hope this helps. Please leave a positive vote if it did, i would appreciate that as it lets me know whether i helped you guys out or not.