Hey there,
I am not a pro painter either, but I painted many many RC bodies. My advice is to just ignore those stickers and do it all with liquid mask. I say the same for headlights and grills too, they are much nicer when you paint them.
About the liquid mask or window stickers:
First, when you get better you probably want those window frame things painted, so you end up cutting up those stickers anyway. Or maybe you want to add other features into the window area as well, like a logo or something. So it's better to go straight to liquid mask.
Second, you get a better fit and crisper line with the liq mask, given that you have a steady cutting hand. You can choose your own exact shape.
I can agree with what lincpimp said about the 3m masking tape. Worth a try if you want to use tape or stickers to mask your bodies.
Last but not least, cutting your own windows is a good cut exercise. Especially the top line of the front window
cheers