Instructions
Enter the building on the right located at the top right portion of Route 3. Route 3 is north of Straiton City. This is the Day Care center. It stands apart from the school on its left by having a fenced area where tame Pokemon are playing with each other.
Enter the Day Care and talk to the woman behind the counter. She will ask to take care of two of the Pokemon in your party while you continue your adventure.
Select one or two Pokemon and she will raise them while you are away. The amount of experience the Pokemon will receive depends on the amount of footsteps you take while they are in the Day Care.
Return to the Day Care after a lengthy adventure and talk to the woman to receive your Pokemon. She will tell you if they have leveled up or created an egg while you were away. You will have to pay a fee for each level your Pokemon have grown to take them back.
Create a Pokemon egg by leaving a male and female Pokemon from the same egg group at the Day Care. If they made an egg, the old man standing outside of the Day Care will tell you when you talk to him. For example, if a male Cradily and a female Corsola are left at the Day Care for an extended period of time, you will receive a Pokemon egg containing a young Corsola. This is because both parents are in the same egg group.