When using an acronym for the first time, it must be spelled out. That's a given. However, if there is a "Definition" section where ALL acronyms that are going to be used in the document are defined and spelled out and this section precedes the rest of the document, do you need to use the full name and its associated acronym later in the document? Example: 1st paragraph of the document explains the document's purpose. In this paragraph, there is an acronym used as follows - "....Delegation Oversight (DO)..." "DO" is now identified as representing Delegation Oversight in the rest of the document. The next section of the document (which follows this first paragraph) provides definitions for all important words used in the document and provides their acronym (where applicable). Following this section is the rest of the document. Do we need to spell out each acronym or can we just use the acronym itself given that it is already defined? My thought is, if the acronym is already defined by the "Definition" section, it does not need to be redefined and spelled out at any point later in the document. Now, if this definition section was to be say on the last page of the document (like an index), then I would think that all acronyms would need to be spelled out the first time they are used in the document. Are these assumptions correct?
A "Definitions" section, like a glossary, can be an extremely useful resource for readers. It doesn't follow, however, that readers will (a) read and thoroughly master the content in such a section before proceeding with the rest of the document, or (b) appreciate having to page back multiple times through numerous intervening pages (if the document is long) to find the appropriate definition each time a term makes its debut in the main section of the document. Some portion of your readers may not be aware of the "Definitions" section at all, if they are reading only parts of the document or if they forget that they bypassed the "Definitions" section at the outset.
As Edwin Ashworth notes in his comment, different publishers have different rules for handling subsequent appearances of terms that appear initially in a "Definitions" section—and those rules tend not to be negotiable. Nevertheless, it makes sense to put yourself in the reader's shoes and judge whether the convenience of including a same-page definition when a term first arises in the main body of a document constitutes a significant benefit to the reader, notwithstanding the existence of the reference section.
It's a judgment call, of course, and it depends to a large extent on the length of the document, the complexity of the terminology you're using, and the level of familiarity that you can confidently expect a typical reader of your document to have with that terminology. But making the right call will amply repay the effort you put into analyzing the situation.
Where: The first time the words are spelled out, is also the first time the acronym should be used. It makes for a smoother, consistent flow for the reader when it's clearly defined up front. Searching for its definition buried 2 or 3 paragraphs down is annoying. If there are other acronyms within the document, the same use of "where" also applies.
How: The proper introduction of an acronym is spelling out the words, immediately followed by the acronym in parentheses.
Of note: I've seen it used in the opposite way, i.e., the AP (Associated Press) picked up a story; and the correct usage, i.e., Associated Press (AP) picked up a story.