I want to practice my singing but I can't do it properly because I have neighbors; my question is, would it work if I made a practicing cubicle with soundproof all around it? I am not sure if soundproof blocks sound going out as well, so if you confirm that for me as well please.
Depends what kind of soundproofing but for singing maybe you'd want somethign with a bit of reverberation (ie not completely dead sound) ?
If so then maybe something harder on the inside layer to reflect some sound back at you, then a layer outside of thick soundproofing panels. That would work both ways - stop sound going out as well as in, but the reflective surface inside would mean it'd probably stop mid-higher pitched frequencies getting out more than getting in (which is what you want). Bass just rumbles through everything unfortunately but although there's a bit of bass in the human voice, it's not much compared to a bass guitar etc.
To be able to practice while being confident of not bothering your neighbours, you'd probably need quite an efficient soundproof cubicle so make sure it's sealed properly at all the corners/joints etc - think of it like a bucket of water : any holes will let the water (sound) out.
Have you considered finding somewhere like in the middle of a field/park or something to practice ? Or a studio ?
There are DIY plans available online to build a "practice cube," or if you're rich you could buy a commercial one :-) . see for examples,