First of all, let us remember that "fitness" is defined as living enough to reproduce. If an organism has survived long enough to reproduce, it has done its part as far as survival is concerned, and id deemed "fit".
1) There is a struggle to survive. LIFE IS HARD. Survival is affected by limited resources, predators, and environmental factors. Organisms must compete with other organisms to obtain the necessary resources to survive. Organisms must evade predators and find successful ways to survive in challenging environments. Many more offspring are produced than those who stay alive long enough to reproduce themselves. Some offspring survive to adulthood and reproduce, many do not.
2) Traits in offspring are inherited from their parents.
3) There is variation in these inherited traits in the offspring. Most individuals among the offspring will not be identical to each other or their parents, but at the same time will carry the traits of their parents.
4) As conditions change, variations in traits may become more favorable than others for survival. Individuals who have the traits that enable them to survive and reproduce will pass these traits down to the next generation. Go back to step #1.