Many UST makers use a technique of creating a midi in FL studio on top of the original song to match the vocals, then importing it to UTAU to add lyrics/tuning. I do it, as well as EchoBlossom and SkyIsland, and probably many more.
People who have Vocaloid3 and are used to it will probably do the same type of thing by importing the song on one track, and copying vocals on another. They export the VSQ and import it to UTAU, or leave it alone if it's going to be a vocaloid cover anyway.
For more specific help, mind explaining what you mean by "they all sound horrible"?