Just take any bus from Al Sadd to the the Bus station. I know bus routes numbers 31 and 32 operate on this route but there also a couple of others. From the main bus station take the bus to the airport and I guess bus route 747 will take you to your destination. You may check on this at the Karwa office located at the bus station. I believe there are no direct bus service from Al Sadd to the airport.