What does Ralph tell the boys they must do to facilitate rescue and what changes had to be made as a result of the fire?

What Ralph tells the boys they have to do is have a fire that will make smoke.  If they can do that, then a passing ship or airplane could see the smoke and know that there are people on the island that need to be rescued.

When they first make a fire, it gets out of control and one of the littleuns is killed.  Because of this, they have to make changes.  They decide to have the fire in a given place and they decide to have people assigned to take care of the fire all the time.