Choose a POD printer. Dozens are available. Do a quick Internet search to turn them up. Choose one that meets your particular needs. Some offer design services and will layout your book and design your cover for you. Some will automatically make your book available online, others won't. Some are geared to single-book authors, others to small publishers who don't have their own printing press. Create Space (previously called BookSurge), Lulu, and Lightning Source are three of the largest.
Lay out your book. Use the information your printer provides to set up your book size and margins. Look at other books in your genre to see how professional designers handle handle font size, chapter header style, page number placement, etc. Choose a look you like and then reproduce it.
Design a cover. Use the dimensions your printer provides. Again, look to see how the professionals do it. Combine elements from three or four covers you like to come up with something that suits the image of your book. Experience with graphic design software helps at this point. If you don't know how how to use Photoshop, Illustrator, or something comparable, you will probably need to hire someone to design your cover.
Convert your cover and interior to the correct format. Most POD printers want the inside of the book in Portable Document Format (pdf) with embedded fonts. Convert Word or InDesign files to pdf using Acrobat or one of the other pdf converters available. Submit your cover in the format requested by the printer. They will typically ask for either pdf, tif, or png.
Submit the files electronically to the printer. Provide them with the author's name, book title, book description, and any other information they ask for. Check the online preview and then order a printed proof.