There is a local bakery that is accepting decorator internships. I don't have any bakery experience but a job in the bakery at Winn Dixie about 15 years ago. I am a beginner decorator and I have taught myself what I do know. I know how to make MMF fondant, rolled it out and cover a cake pretty good, colored fondant and cut out the decorations , made some great buttercream icing, and have taught myself lots of the Wilton techiques with the help of these message boards, other websites and you tube.

What I need help on is how to say all this in a letter (or email as they ask)?

I think you just explain that you are largely self-taught and eager to learn more skills in a professional environment. Basically just organize what you have just indicated here explaining that you have used various resources to learn new techniques and have implemented your skills as a hobbyist. Internships and apprenticeships in any field are designed for people with limited/beginner or even no prior experience or skills, so your eagerness and interest along with base knowledge and experience should more than suffice. Maybe also provide photos of some of your work if you feel confident to do that.
That's just my opinion.
Good Luck! Sounds like a great opportunity.