Instructions
Win against the first four gym leaders in Johto to obtain your first four gym badges. Travel through Route 41 to get to Cianwood City for the first time. Go to the west side of Cianwood City to find the Safari Zone.
Speak to Baoba, the Safari Zone Warden. Give him a Geodude and a Sandshrew and he will allow you to customize the Safari Zone with various items. These items will attract new Pokemon to the Safari Zone.
Get the National Pokedex from Professor Oak in Pallet Town of Kanto. Obtain 10 Forest Objects, such as Trees, Stumps and Branches. Get 14 Peak Objects, like Small Rocks, Big Rocks and Mossy Rocks. You can get these objects from Baoba.
Decorate the Meadow section of the Safari Zone with the Forest and Peak Objects. Once you have all the objects situated on the Meadow, wait at least 70 days. After 70 days, Riolu will appear in the Safari Zone at Level 45.
Walk around in the long grass of the Meadow. Run away from every Pokemon until you come across a Riolu. Throw "Mud" at the Riolu to weaken it. Toss "Bait" at the Pokemon to make it less likely to flee. Use your 30 Safari Balls to capture the Riolu.