This guide may be used by beginners to experts in any version. I have become quite familiar with this because I have studied this same process for an extensive full month. Through many hours, here are my findings:
What is Breeding? Breeding is the mating of two pokemon of opposite gender to produce the desired offspring.
Where can Breeding be done? Near the road between Mauville and Vendanturf (various versions) At that day care. Make sure you have 200 coins though!
Who does Breeding? Breeding can be done by pokemon of the opposite gender, for example, a female would have a male and vise versa. And they must be of the same type of element. For example only a water type can mate with a water, a fire with a fire, a steel with a steel, etc. Along the way you may notice 2 pokemon may not get along well. In this case you must find a suitable FEMALE partner (because she is the only one that can produce eggs) or any same element, opposite gender male for your female.
When can Breeding be done? Breeding can be done at any time you want as long as you meet the requirements.
Why is Breeding done? Breeding is done to produce the desired offspring of the parents.
Below is a step by step procedure so you may be able to do this too.
Step 1: Lets say my name is Palzone and I want to breed two water type pokemon of opposite gender. The pokemon I choose is a female Tentacruel, lets call her A and a male Tentacruel, let call him B.
Step 2: Upon arriving in Mauville I walk west. Or east if I am in Vendanturf. Until I reach a Day Care. And I talk to the lady inside she asks if I want two of my pokemon raised. Agree. ALWAYS, put the female first followed by the male. Then walk outside and talk to the Day Care Man. He'll tell you if both of your pokemon are doing well. If they prefer other pokemon try to pair up that pokemon with another suitable partner. If they our both the same pokemon they should get along very well. Wait patiently and come back in about 20-30 minutes. Your pokemon should have produced an egg which the Day Care man will give you if you talk to him. Now look below to see the 3 stages of the egg while waiting for it to hatch.
Upon reading its summary it says this "What will hatch from this? It will take some time." That means it was only recently released from the Day Care and probably take about 30-40min to hatch while playing the game.
"It moves around occasionly, It should hatch soon." It will probably take 20 minutes to hatch.
"I can hear its sounds. Its about to hatch!" It will probably take 5 to 10 minutes to hatch.
You don't need to do anything special just go on as usual to make it hatch. You will go "Huh?" The pokemon that hatches from it will be the lowest form of evolution. For example, two Tenticruels offspring would be a Tentacool or Charizards would be a Charmander. Since pokemon don't have genes or allelles their gender does not rely on x and y chromosones nor traits. It relies on probability. If you catch a pokemon thats usually a female its offspring will probably be a female and vice versa with a male. Keep in mind certain pokemon cant mate for some reason. So, that: I don't know. But for the rest I hope this guide has helped you and good luck to you on your breeding!
One last question. Where can I get those evolution stones. I know I need to dive for shards in Sapphire for the hunter. But where can I find those shards???