Google street view is now available for Bangkok and so I have extracted these links and views to assist you to find your way to the U-Baan Guesthouse. These street views will give you valuable landmarks to find your way.
The address and phone number of U-Baan is:
961 Soi Rat Ruam Charoen
(Via Soi Krung Thonburi 1) Klongsan
p. +66 88 499 1503
If you are coming from the airport you could take Airport Link till Payahtai station. Transit at BTS Payahtai station to Siam station (center station). Change to Silom line at same floor to BTS Wongwaityai station (end of the line of silom line). Get out at Exit 3 get down at the stairs on left hand side you could take the elevator to get down. Soi Krung thonburi1 will be in front of you.
When you exit BTS Wongwaityai Station you need to see Khlong Ton Sai and the image below shows you a large blue, yellow and red sign which is the first landmark.
Walk along Khlong Ton Sai until you see the Seven Eleven Store. Take the next turn on your right which is Soi Rat Ruam Charoen.
Walk along Soi Rat Ruam Charoen until you come to U-Baan which is on your right and shown in the view below. The number is clearly marked on the building.