Take off laces and clean shoe with cotton balls dipped in nail polish remover. This is THE MOST IMPORTANT step in customizing. As you rub the doused cotton balls over the shoe, you will notice that there is a clear build up that comes off the shoe-thats the idea. Essentially, this step is to remove the "clear coat" that most manufacturers spray on their shoes to protect them. It's a must to remove this so that the paint will bond directly to the leather as opposed to the clear coat.
If the clear coat is left, chances are that the paint will crack after some wear.
I like to go over the shoe at least 3 good times. If you want to go that extra step, use doused q-tips to get into the hard edges and sections.