Working from an outline is the simplest way to draft an order letter. You have already organized yourself by creating a list. Refer back to it and turn each fragment into a full and complete sentence expressing a single thought or idea.
In order that your thoughts and ideas are conveyed in a cohesive manner, write in as natural a sounding voice as possible. Try writing your draft quickly and then read it out loud. Concentrate on communicating your objective to your reader. Make sure that the scope of your letter contains all the relevant information included in your organizational list.
Keep in mind that you are writing a rough draft. For the moment you can ignore spelling, grammar, punctuation, sentence and paragraph structure. Those are technical details that you will pay attention to in the final step when you review and revise your work.