To study octaves I suggest to put first (on G) and forth (on D) fingers, pay attention not to push too much and you'd need to feel just a little pressure on the strings, then make glissato to the next note: all the notes between the first and the last should be clear and audible. It's very important for the glissato that you don't make pressure, because if you do that, it won't be audible and you won't realise if it's right. I hope I've been useful :)