The letter can be very simple, you don't need to go into long detail.
I'd say to quote the law just so they know that you know what you're talking about. My letter would go something like this:
[quote]On XX/XX/XXXX I received your notice of intent to assign my wages. You should know that the Federal Credit Practices Rule (16 CFR 444.2) governs your use of wage assignments and provides that I may revoke it at my own will.
Accordingly, notice is hereby given that I revoke the wage assignment I executed on (insert the date you took out the loan).
Like Fins said, give a copy to your HR or payroll so that they know you revoked it.
I didn't mean to scare you by mentioning lawsuit. Although that is a possibility, creditors will only do that as a last resort. They would most likely only file a lawsuit if you sent this revocation and then never paid anything again.
Believe me, they want it to be paid off without going to court just as much as you do. The revocation just lets them know you're going to do it on your terms. If you contact them and make some sort of payments, I think you will be safe. Ask for a manager, explain your situation, let them know you're not just refusing to pay, but need a little more time.
We have all been there sometime or another, and it is indeed a learning process. Let us know how it goes.