There are a couple ways you can generate a base OTO for your voicebank.
OREMO: If your voicebank is entirely in hiragana and recorded to a guideBGM, you'll have pretty good results with OREMO. Go to Generate oto.ini > Type of utterance > VCV. Set the recording tempo, initialise parameters, then set the Utterance Start to where the first syllable of the sample starts MINUS the preutterance value.
Moresampler: Compatible with both hiragana and romaji, though there are a few rare syllables that it tends to misread. Drag and drop the voicebank folder onto moresampler.exe.