Instructions
Give the Soothe Bell to Riolu. Battle trainers with the Amulet Coin or sell your unneeded items, and use the income to purchase vitamins to feed to Riolu.
Give Riolu a massage at Veilstone City whenever possible. Avoid feeding Riolu bitter food such as the Revival Herb or letting it faint in battle.
Repeat steps 1 and 2 until two large hearts appear next to Riolu in the Poketch's Happiness Checker. Raise Riolu one level during the daytime to evolve it into Lucario.