A .UST file is a file that UTAU reads to make your UTAUloid sing. Many people who make these will share them with you. When you've downloaded a UST file that you would like to use, you simply open UTAU, go to the "File" tab, select "Open" and search for your UST file.
Then, simply click and drag to highlight the section of the UST that you would like to play, then hit the play button. A loading screen should launch, then you should hear your UTAUloid begin to sing the UST.
However, if you wish to create your OWN UST files, there are several ways to do this. One of course, is to make it by ear, however, there is an easier way for beginners. You can import a MIDI file into UTAU, and then add lyrics, pitch bends, and vibrato.
Here is a quick visual step by step.
So those are the very basics to making your own UST file. If you need more in depth help, there are lots of video tutorials available on YouTube.
Once you are done with the UST, it's time to export the vocals. To do this, simply click the "Project" tab, then "Export Wave" then choose where you want UTAU to export the vocals to.
