This piece - the 7th from '70 Easy to Intermediate Pieces' - requires the lowering of the lute's 6th course by a tone; this retuning was fairly common in the 16th century, partly because it provides a useful extra bass note, but also because it creates a wonderfully resonant drone on the open 4th, 5th and 6th courses. This is exploited to good effect in many dance pieces, of which this Volte is a good example.
The first issue is, of course, the retuning itself. I suggest that you start with the upper octave of your 6th course, detune it by much more than a tone, (a major third will do), then bring it back up to make a unison with the fundamental of your open 4th course. Next, lower the 6th course fundamental by a similar amount, and bring it back up to make a clean octave with its partner. Both strings will now hold their new pitch more readily than if you merely lowered them by the required amount. Next, check that you can play the detuned course with a decent forte thumb stroke without the course rattling. If it does rattle, you may need to put on slightly heavier strings or pluck a little closer to the bridge. Sometimes pulling the strings up at the bridge by hooking a finger under their loops can solve a minor rattle.
Once you have the lute settled to pitch, the first technical challenge is for the right hand thumb, which plays the drone bass all by itself - 6th course, 5th course, 4th course, in each and every bar. It may appear obvious to use a rest stroke with the thumb, in which case the only difficulty is negotiating the return at each barline. However, I find the combination of a drone bass and rest strokes can be somewhat relentless and too heavy for the melody, so I prefer to play this with free strokes in the bass, saving a rest stroke for the beginnings of longer phrases. This demands a light and accurate touch with the thumb, and I suggest that you practise the bass part on its own until you are happy with the result. Start slowly, and gradually increase the speed, using a metronome if possible to ensure steady rhythm. Be careful that you don't take too long to replace the thumb at each barline.
Once the bass is flowing smoothly, add in the downbeat melody notes, using the right hand middle finger to pluck them. The melody needs to ring out confidently over this strong and fairly busy bass, so make sure there is plenty of contact of finger on string. Listen carefully to the chord that results; it makes little sense to spread the two-note chords in this context, but many an inadvertent spread is created by imperfect technique. If your treble and bass notes are not exactly simultaneous, practise these points until they are. For practice purposes you can simplify the melody by playing only the first melody note in each bar, but with the whole bass part.
The final stage is to put in the remaining melody notes, those on the third beats of most bars. Ideally, these should be plucked by the index finger, since they are all on weak beats. The same problems of simultaneity may arise, and can be ironed out by slow practice with just the affected chords (using thumb and index to pluck them - don't be tempted to replace the index finger with the middle finger when the chords are out of context).
Once you have achieved these technical necessities, put the whole piece back together, and work on building up the speed. A Volte should go quite fast - more one in a bar than three in a bar - and the right hand must remain relaxed in order to achieve this.
As a reward, and in order to make a more substantial piece to showcase your hard work, I suggest you try improvising variant melodies over the same drone, taking the shape and rhythm of the existing melody as your model. You can also introduce repeats at the halfway point and at the end.