If you are like I was, it will go against your usual habits to resist trimming. Because facial hair is cut at a 90 degree angle to the shaft each time you shave, when it starts to grow and make contact with the skin, it will be scratchy. The natural tendency will be to cut off the offending hairs--especially the ones that annoy you by itching your lips or the corners of your mouth.
Here's the thing: you may need these hairs later and should not trim anything until at least two or three months in when you can positively confirm the offending hair is truly useless. You want to have as many hairs as possible to work with once you get you moustache up and running. Trust me on this one.
The worst thing you can do is to trim along the lip line like you do when maintaining a goatee or short cropped moustache. This will give you a mullet-moustache. No good. Think of your hairs as chess pieces--would you purposely sacrifice every one of your pawns for a short-term gain at the beginning of the game? You wouldn't. Those pawns may become powerful players someday. Stay strong!