Our flight to Barcelona was from Treviso Airport about 40 minutes away from Venice (cheaper airlines like Ryanair fly out of here).
Instead of waiting in lines at the bus stations you can buy tickets from the tourist office at the Santa Lucia train station; the guys there were very helpful and even had a timetable of bus departures handy. The tickets are for the ATVO airport transfer bus and cost 7 euros one way.
Then you head to the bus lot at Piazzale Roma and find the parking spot for the airport busses. Be careful not to get on the wrong one as there are multiple airports, for the most part the spots are marked accordingly. It is good to get there early because the schedules never seem to be up to date but they try to coordinate the schedule with Ryanair flights. Also, busses can fill up quick depending on the time of year. When you’re on the bus just put your ticket in the little yellow punch box and you’re good to go.