Use an audio editor software and follow these steps:
1. Make sure the tempo marked in the sheet music matches the tempo of the midi. If the midi is too fast or too slow, adjust it accordingly.
2. Listen to the midi several times while looking at the music (not singing yet, just getting an intuitive feel for the rhythm and notes).
3. Sing through the piece, working out difficult notes or passages with a piano, until you can sing through the whole piece smoothly.
4. Apply optimal mic settings, then try recording (with the midi playing in your headphones). It might take several tries before you nail it. That’s ok! If your voice is getting tired and you’re feeling discouraged, drink some water and take a break for an hour or two. Then try again.
5. Put your vocal recording alongside the midi track in GarageBand, or your application of choice.
6) Listen to the recording with midi. Watch the sheet music for any faulty notes or rhythms.