The best way to the airport is straight down Bell Street and follow it all the way down to the Tullamarine free way.
If travelling by car, your best option is perhaps to head north toward Bell St, turn left onto Bell St and follow it all the way through to the Tullamarine freeway. While it depends on traffic and the time of day, Bell St is not too congested when heading west and you should be able to get to Melbourne Airport within 30 minutes.
The most direct route would be to get onto Bell Street and head West and then its a right onto the freeway to the airport,generally takes around 30 minutes depending on traffic conditions
We also take Bell St to get to the airport. Bell St. can be a bitch though, so you sometimes have to take the side streets, parallel to the left of Bell St.