Parts List:

• Brick 1 x 8 (4)
• Brick 2 x 4 (2)
• Brick 2 x 6 (2)
• Brick 2 x 8 (2)
• Technic Gear Rack 1 x 4 (8)
• Tile 1 x 8 (2)

Notes:

• This is where that "having more pieces than come with the Mindstorms set" thing comes along.  This step involves 8 of those "gear racks."  Your RIS sets came with only 4 of these.  If you don't have the 12 required, I'd advise you not to continue.