Instructions
Travel to either route 111 or route 113. Walk around in the deep sandy areas in either of these routes until you encounter a wild Pokemon that is a Sandshrew. In any given encounter, you will have a 35% chance of coming across a Sandshrew.
Weaken the Sandshrew by attacking it with one of your Pokemon; once it is weak, throw Poke Balls until you catch it.
Use your new Sandshrew in battles to make it gain experience points; once it reaches level 22, it will automatically evolve into a Sandslash.