>>6501Dreamtonic's synthv intro tutorials are a good place to start. The good news is that you've chosen one of the most user friendly English-language synth programs available.
In general I would recommend a workflow like
>Import your .midi vocal melody>Input lyrics>Edit phonemes (individual word sounds) and their timings to tweak her accent and put emphasis on the right notes>Edit pitch as necessary - Teto especially needs help sometimes with her vibrato>Edit her voice color (power/cute/etc) as necessaryAnother trick for Synthv is to use the "br" and "cl" phonemes (breath and glottal stop, respectively) to produce more natural results.
Play around and have fun! Hope you enjoy.