Begin with an idea. Then decide what kind of manuscript would be best suited to your idea. Perhaps your idea isn’t complex enough for a book, but would be perfect for an article.
Continue with an outline. Many writers use some type of outline to keep their ideas from getting unruly; others write as thoughts come to them. An outline can also be used to measure your progress as you write your manuscript.
Write! Put pen to paper or fingers to keyboard. It sounds obvious, but good intentions don’t write a manuscript. If you can't seem to find time to write, you may have to schedule it. Whether you write for 15 minutes in the morning when you feel fresh, on the bus on your way to work, during breaks or before you go to bed, a little writing each day will eventually produce a manuscript.
After you finish writing, set your manuscript aside —ideally for at least 24 hours. This will give you a fresh perspective. Then begin revising. You may have put a lot of blood, sweat and tears into your manuscript, but you must be brutally honest. If something doesn’t work, either revise or delete it. Warning: Don’t fall into the trap of never-ending revisions. Go through your manuscript from beginning to end another time or two if you wish, but then stop. If you don’t, you’ll never move on to the next manuscript.
If you will be submitting your manuscript to a publisher, make sure it’s formatted properly. Nothing says amateur like triple spacing. Review publisher guidelines. If in doubt, use 12-point Courier font, double-space your manuscript and use 1-inch margins all around your text on every page.