Milandro has given some good video's to help you along. A few point of advice however:
<disclaimer>I have never actualy done such a conversion, but I did take a course in noise pollution as part of my education a an environmental expert. And it's a matter of interest for me, I have read quite a bit about how sound behaves</disclaimer>
Themal isolation will not help much to dampen the sound, if you need the thermal isolation it will help a tiny bit towards sound isolation (very little actualy).
What you need is dampening materials that have a rather high mass (like high density rock wool) to "capture" the sound vibrations. And a suspended construction (as demonstrated in the second video) is very good.
Some of the materials used in that video have more to do with the esthetics than with the accoustic aspects. Like the plaster boards that are used, I think that dampening materials without the plaster front will work better.
One of the most difficult sounds to capture are the ones transmitted by contact (through walls, floors, construction beams, .....). So the best way to make sure you don't let any sound out is by using suspended walls, suspended floor and suspended ceiling.
And you could also check out this: https://blazersenblazers.be/nl/studio-bricks
but that is quite expensive. Still, it works very well, you could go and check it out in Ghent.