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.
Ralph has hope that the boys will be rescued, but he is aware that they need to have a plan for rescue. He develops a plan to light a fire and keep it burning. However, the first fire is too large and gets out of control damaging part of the island. They then decide to build one in a different location and try to maintain it as smoke not flames. In addition, Ralph tries to get it set up so that the boys will rotate keeping the fire burning but he does not have much success.