Where? Not on your own house or your mums car atleast. When? Whenever the situation allows for it. Why is this so hard?..
If you don't know how to sneak out of the house at night, and cant tell your parents what you're doing then you need to just draw on paper and wait until you live on your own to even buy a can of spray paint.
Option number 1 is sneaking out of the house with the cans, if they have been mixed properly you might want strong magnets to keep the balls still. Put the cans in something soft so they dont clank against eachothers Option number 2 get the cans outside first, such as put them on a windowstill and come get them from outside or hide them outside
Thanks my parents actually found out I tag because they came home and I was mixing paint in the sink and they thought I was sniffing it so had to tell them. They don't really give a shit tho.
I dont know how long youll be living with your parents for, but im gonna go ahead and make the assumption that youre pretty new to doing graffiti. My suggestion is to stick with paper and really develop your style. If you wanna go out and bomb, either just use markers which you can hide pretty easily, or only keep a few cans of paint at a time. If youre buying Rusto, stash it with a toolbox or some shit to make it seem purely practical. There are ways around parents, you just need to be smart about it until youre living on your own.