Whether they're doing it for charity ("Movember") or just for fun, you need to read this before growing a mustache. They required a lot more upkeep and care than stubbles or fully shaved looks.
Think of a shape in your mind, and imagine it is covering all your lip right now. Yep, your mustache should go farther than the end of your lips that is wide enough to be able to carry it. We suggest to trim to your desirable length using an electric shaver fitted with a guard attached.
You'll have to shave regularly to keep it neat, with distinction between your mustache and the rest of your facial hair apparent. Remember this isn't like stubble where you can wait a day or two in between shaves.
Put a bit of beard oil sporadically if the skin underneath gets scratchy or itchy. If it start to look unkempt, try to brush it down with your fingers or a comb.