Make your way to BTS Ari and leave by Exit-3.

 

When at ground level, walk a short distance going north, then turn left into Phahon Yothin Soi-7

 

Walk about 300 metres up Phahon Yothin Soi-7, then turn right into Soi Ari-4 

 

The A-site is a short distance down Soi Ari-4 on your left.

 

You'll find a 7-11 shortly after turning up Pathon Yothin Soi-7, if you need some water.  Bring a torch, since it will get dark!.