Instructions
Raise your Riolu's happiness level. This is required for it to evolve. Raising happiness is done by giving Riolu healing items or taking it for a massage in Veilstone City. Give Rilou a Soothe Bell to hold, if you have one, and then walk around to steadily raise its happiness.
Check Riolu's happiness to see how close it is to evolving. Do this by tapping on Riolu's icon on the touch screen. Hold the stylus down on the icon. If you see two large hearts, Riolu is close to evolving.
Play the game during the daytime. If Riolu is happy enough, it evolves into Lucario. Riolu won't evolve if it is dark outside.
Go to the Pokemon Mansion and talk to the owner. Continue talking to him until he says that the Pokemon Ditto is in his garden. Go out to the garden and capture a Ditto.
Take the Lucario and the Ditto to the Daycare Center. Leave both of them there for a few hours. After a brief waiting period, the Pokemon breed and an egg appears.
Keep the egg in your party as you walk around. When the egg hatches, Riolu comes out. You are able to repeat this process to get as many Riolus as you want.