If you are on a mac, it is very easy. Make a new garageband project, and drag the .mov into garageband. Now with the movie track selected(should be the color purple), hit cmd + delete. You say the .mov is trimmed down how you like it, yes? Go to SHARE -> SEND RINGTONE TO ITUNES. It will ask you to adjust the length, so click on ADJUST. You will see a yellow bar at the top of Garageband. At the beginning and end of that bar, you can left click and drag to adjust the length of the ringtone. Once set, again go to SHARE -> SEND RINGTONE TO ITUNES. This time it will work(assuming the length is under 40 sec), and iTunes will open. Now, right click the ringtone you just made, and click on GET INFO. In the INFO tab, you can change information like Ringtone Name, ect. Hit OK when you are done. Now just plug in your iPhone, and you will be done.