If you take BOLT Bus, you'll get off at the Amtrak Station ("30th Street Station"). Go inside and ask for the location of the Airport trains. Take 1 to the terminal stop closest to your departure. If you take Greyhound, after exiting the Bus Station (10th and Filbert St), walk across Filbert Street and enter the MARKET-EAST Septa STATION (Big Signs - you can't miss it). From there, ask a Septa employee for directions to the Airport Trains. Here's the schedules for both 30th Street and Market East: http://www.septa.org/schedules/rail/pdf/air.pdf
Fare: $6.50 One Way from either Location.

There are lot's of trains in both of the above stations, don't get on the wrong one!
Of course you could easliy take a cab to the airport directly from either of the above Bus stops. The City Regulated FLAT fare from either location is $28.50 + tip, regardless of how long it takes. Make certain the cab driver confirms the Flat fare, before you go - some are to put it diplomatically, forgetful.