This question came up recently—again!—so I thought I'd answer it here for posterity. TFor manuscript formatting (suitable when you're preparing the document to be submitted to contests, to agents, to acquisitions agents and publishers) use standard manuscript format. William Shunn is a bit of a codger: no one uses Courier anymore (Classic Format). Use Times New Roman 12-point instead. And usually, 1.5 line spacing is okay, in Modern Format... but if in doubt, and especially if you're submitting a printed manuscript, go with full double spacing (2.0 lines). Note the variations for short stories and other things intended for publication which are also found on Shunn.net.
For freelance articles, begin in standard manuscript format but always always always check each publication's submission guidelines and obey them.
Poems (considered individually), chapbooks, and poetry collections are a bit different.
And if you're preparing a document to be a source file, to upload to KDP or Smashwords or IngramSpark or something, that's a whole new challenge. I'd go with the standard manuscript / poetry collection manuscript format during the revision, editing, and proofreading process, for two reasons:
- It is super easy to revise and edit (that's the whole point of standard manuscript format)
- Book designers and professional formatters like Wordsmith Writing Coaches partner Carla Green expect manuscript format, so if that's what they get, it makes their formatting job much easier and quicker, which saves you time and money.
But if you want to format that source-file document for publication yourself, this is a great learning opportunity. Begin with the classic book Formatting e-Books for Writers and practice on your own manuscript! In this book, Susan doesn't limit her focus only to ebooks; she also describes how to format for the KDP/Amazon print-on-demand meatgrinder... which happens to be a close cousin to the Kindle meatgrinder. If you do your formatting yourself, remember that the order in which you submit your source file (to KDP and Kindle, or vice versa) makes a difference: if you upload it to KDP and KDP accepts it, KDP can later translate it to Kindle without much fuss, as long as you don't have images or diagrams in your book. But if you format it only for Kindle and upload it to Kindle first, you'll need to re-format your source file before you upload it to KDP, to add things like page numbers and to make sure the pagination is correct.
There are also amazing apps like Vellum that can do book formatting for publication (changing your manuscript to a publishable source file), but they come with a significant learning curve. Whether you do it yourself or learn to use an app, you'll have a bit of a hill to climb. But it's a lovely view once you reach the top. Here's a positive review from one of Vellum's competitors.
I hope this is helpful! Let me know in the comments. Updates and corrections welcome!
("Meatgrinder" is slang for an algorithm that aggressively transforms the way you've carefully formatted a book, and yet requires you to submit your document in a very precise format. ...Yes, a very precise format that will be instantly destroyed and regurgitated in yet another format that Kindle readers and ebook distributors can handle.)