From Suvarnabhumi Airport, take the Airport Link to Makkasan station and change to the MRT subway to Hua Lamphong station. Then, take a taxi or an air-conditioned bus, number 40. It's a starting station that passes through Wongwian Yai and Charan Sanit Wong Road. Expenses are around 20-25 baht. We do not recommend this route because it takes a lot of time, but if you have a lot of time and want to get a new experience, it's okay for you.
From Suvarnabhumi Airport, take the air-conditioned bus number 556. They service around 06.00 AM—09.00 PM and cost around 24-35 Baht.
If you stay in the Sukhumvit area, you can take the air-con bus number 511 (orange color) to the Southern Bus Terminal. Expenses around 20 baht and inform bus staff to drop off at this station (last station of bus route). Take a time of around 45 minutes or more. (Depends on traffic.)
If you stay in the Khao San Road area, a taxi is the best way because it's the fastest.