After you have defeated Kraid and Ridley and gotten the Screw Attack, you can continue to Tourian.

Explore Tourian

There will be a cutscene when you enter Tourian. Go through the door on the right. Save the game if you wish, then go through the door on the right. Go past the dead Space Pirate and go through the door on the right.

There is a turret on the wall on the right. You can shoot the rings of light that it shoots out to destroy them. Go down to the bottom of the room and go through the door on the left.

The door will lock behind you. Go to the left and a Metroid will appear. If it latches onto you, use Bombs to shake it off. Shoot it to freeze it, then shoot missiles or charged shots at it to kill it. The Metroid thaws out quickly, so keep shooting it to refreeze it as needed.

At this point, the path is straightforward. Kill Metroids as you go. Eventually you reach a save room. Save there if you wish (it's not a point of no return), then exit the save room and shoot the floor. Go down below, and go through the door on the right.

Make your way down this room, killing Metroids, and open the door at the bottom. In this room, the door on the left is locked. Bomb the floor to open it, and kill the Metroids you find down below. Bomb the next floor and kill the Metroids below there. Then the upper left door will unlock. Go through it.

Save in the save room (it's not a point of no return) and go through the left door. Go through the room, shoot the left door with a missile, and go through to face Mother Brain.