Here's the step by step instructions courtesy of One Good Thing By Jillee
Fill the washer with HOT water. Add 1 quart of chlorine bleach - no detergent – and let the machine agitate for a minute, then let it sit for one hour.
After one hour, allow the washer to run through its longest wash and spin cycle.
Immediately fill the washer with HOT water again and add 1 quart distilled white vinegar. Let agitate for a minute, and again, let it sit for one hour.
Using the bleach and vinegar will clean away bacteria, soap scum and mineral deposits from the wash basket and hoses. This is especially important if you live in a hard water area and should be done every three months! Every washer should be cleaned at least twice per year.