You can get the hidden ring in Hidden Base by doing a high "hover jump"(which is just jumping and activate the hovering as soon as you can so you'll start to get higher for just a little period of time) by pressing the "A" button and pausing the game as fast as you can(usually placing 2 fingers on both buttons and pushing them at the same time) and later doing the same thing but pressing the "A" button later, this way you'll probably reach it, knowing that you'll still need some speed before your "hover jump" or else you won't make it.
For Green Forest is just a light dash what you need, stay in front of that cylinder then do a light dash, if you don't pass through it then jump before the light dash, this should work for you.
Anyway, you can learn a lot of things by watching videos, there's a video from psyknux showing how to get these rings in Green Forest, and Sadistic Mystic has a video showing the locations of the "BIG" rings, it's on his youtube channel.
