Alright, here's the code. Take the normal alphabet, and line it up over the alphabet backwards. Where'd you'd normally put a letter, put the letter below it in the backwards alphabet. So if an A is 1 letter into the normal alphabet, put the letter 1st in the backwards alphabet (Z).
To solve my code, find the letter in the backwards alphabet, and replace it with the normal alphabet one.
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZYXWVUTSRQPONMLKJIHGFEDCBA
Decipher this: Xmwvh ziv ufm