I am in the National Guard and want to go AD. How should I start off my letter for my conditional release?

I wouldn't start with a letter. I'd start with a conversation with your 1st Sgt and an AD recruiter - of any service, BTW.

You should not need a letter. Speak to an AD recruiter first, then speak to your command and make your wishes known. Most commands are more than happy to oblige. Years back I was Reserve and upon completing AIT I wanted to go active. I spoke to my AD recruiter and my command signed off on it. They handled all the paperwork and I simply had to sign at a few places and then run through MEPS again. Done.

Have you spoken with a AD Recruiter? I’m sure they will know what to do.