For a full explanation of the code, visit http://en.wikipedia.org/wiki/Vigenère_cipher
In this instructable, I'll lay out a method by which the cleartext (the initial message, before encryption) can be disguised to help prevent decryption using either of the methods, since even decrypted it would read as gobbledygook.
The concept of rotating cleartext I got from Dan Brown's book Digital Fortress.
Step 1: Select cleartext
However, you could choose any cleartext you want, you can usually play with it to get a square number of letters.