Choose a digital printer you'd like to use for your book based on your needs and how much you're willing to invest in it. You can search publishing directories online as well as self-publishing sites such as Createspace, Lulu and Lightning Source. Dig deep enough and you may find a digital printer in your area that you can visit to look over the quality of the finished product.
Set up an account with your digital printer by sharing contact and billing information. For self-publishing sites such as Createspace and Lulu, you'll need to use your email address and a password to log in to your account. Note that most, if not all, printers require a Social Security number or employer ID number for tax purposes. This information is not shared with anyone else. It's used at the end of the year for taxes.
Follow the instructions based on the digital printer and the dimensions of your book to lay it out correctly. For example, Createspace and Lulu both have a softcover book with a trim of 5.5 inches by 8.5 inches. In your word processor, change the dimensions of your work by clicking on "Layout" and putting in those numbers. When you hit "Enter," the book will grow in size page wise, but its margins might now be too large. Change the margins as well under "Layout" to what your printer requires. Remember that in the gutter of the book, you want more room so the words aren't stuffed down inside the spine.
Set up your cover work either by using a template the company has given to you, or by following size specifications. Generally, printers will help with this step due to how complex it can become. You may also pay the printer to make a cover for you if you like.
Upload your files to the site you've been using or burn them to a CD for a local printer. If all things check out, you can order a proof. A proof is a single copy of your book in its rough stages to give you an idea of what the finished product will look like. Take a little time to read the book front to back, skimming for any errors you may have missed.
Approve the title for publication once edits have been made. Digital printers move pretty fast, so you should have a finished product within a week or so. The nice things about digital printers is that if later on you'd like to add onto the book, you may do so by setting up new files and paying a small fee. This is so that future publications will be up to date.