Instructions
Go to the Safari Zone on Route 48 to the west of Cianwood City in the far west of the Johto region.
Speak to the character Baoba at the entrance to the Safari Zone. He will give you 30 Safari Balls and assign you two tasks, which act as a tutorial in how to use the Safari Zone. To complete the first task, just go into the first Safari Zone area and walk around in the grass until you encounter a Geodude Pokemon. Throw a Safari Ball to catch it and bring it back to Baoba.
Wait three in-game hours and speak to Baoba again to accept the second task. Go to the computer at the entrance of the Safari Zone, where you can change which zones are available. Swap the Desert map for the Plains one, allowing you to go straight into the Desert zone from the entrance.
Head into the Desert area and catch a Sandshrew in the same manner you caught the Geodude. Bring it back to Baoba to pass the Owner Aptitude Test, giving you full access to the Safari Zone. You can now choose any zone and place a number of objects or "blocks" in them to attract Pokemon.
Go to the computer and move the Rocky Beach area to the front of the group. Place 24 Rocky blocks in the Rocky Beach, and head inside. Walk around in the long grass until a wild Aron appears, and throw a Safari Ball to catch it.