Play the Green Forest Level as Sonic and keep playing until you get to a part in the level with three robots and a gold bomb beetle. You should be able to hear the sound of an oppenent dropping bombs up high. Boost on a speed ramp and you will be able to see another pair of speed ramps ahead of you when you get to the top of the first loop.

Go on the speed ramps ahead and you will do another loop and automatically hit a pair of springs and be bounced onto a big platform with Check Markers leading to the next part of the level. Instead of going through the Check Markers to the next part of the level, move to your right as far as you can go without falling off.

The camera angle should automatically change. You will see the enemy who was dropping bombs. Attack the enemy and you will see a box of rings. Use Homing Attack on the box of rings in mid-air straight after you attack the enemy who was dropping bombs and either use Homing Attack again after that or press toward the way the box of rings faced you after you got the rings in mid-air. You should land on a small platform with a green light on it. That should be the Ancient Light for Sonic.