Here is a link containing the maps.
You'll need HMs: Rock Smash, Surf, Rock Climb. And you might want TM Flash.
First, Surf around until you land on the stairs near exit B. Then, head down toward Exit C. Go down the stairs. Turn right. Turn left and go up the stairs. Go all the way around to exit Exit E. Turn left and go down the stairs. Go up the next stairway, turn down, and go down the next stairway. Then, go to up the stairs to exit D. Go in.
After coming out, work your way through the maze until you reach Exit A.
After coming out of A, go down and head to exit G.
After coming out of Exit G, go around until you reach the stairways that lead to a body of water. Use Surf here. Surf right and up. Get off and work your way to another body of water. Take the first right path you see, and there lies Mewtwo.