The general idea is that you put the original song audio on one track, and a placeholder midi instrument on another track. After listening and copying the melody onto the instrument, export that track as a midi. This is what people do in FL studio, and what you can adapt to the workflow of other DAWs.
I installed MuLab's Mac demo quickly to check out how the process would go. To import audio, drag and drop a WAV file into the window and select Audio Stream. Check the tempo of the song using a tapper like this website:
https://www.all8.com/tools/bpm.htm then double click on the tempo at the top of the window to change it. By default MuLab sets it to 128. Click on the metronome icon (triangle with a line in it) until it's green so that you can hear the ticking, and line up the song so that the ticking matches the beat.
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To add a MIDI track click on the + button in the area on the left and add instrument track. You may want to use the default Piano sound included with MuLab, though you should probably choose a different instrument if the song already features piano prominently so that you can tell them apart. Double click, then click and drag to create a midi clip in the track area.
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Double click on the midi clip to start editing it. At the top you'll see little curved arrow flags. These are the loop points, extend it to the right to match the length of the song so it doesn't accidentally loop the midi in the middle of the song.
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Double click in the piano roll to create new notes, and listen over the notes and adjust them until they match the vocals of the song.
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When you finish this for the whole song, switch back to Compose view and right click on the clip to export the MIDI file.
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