After Hercules, it's probably the most difficult sauna to find, but used to be rewarding in the early days as Uomo(I met two first-time massage boys from Ramkhamhaeng Uni there...when it used to offer massage). You could simply take a taxi and ask for Rama IX road, soi 49, #551. But you'd probably be nearly broke by the time you got there and wouldn't have a clue where you were.
My advice is to take Micro bus 17 from Silom, going east(pay Bht25 into machine upon entry) and sit back and relax till you get to Ramkhamhaeng Road...get off just after you pass the huge Hua Mark stadium on the right. Just after(going away from Bangkok) the stadium(can cruise there late if you can't find anything Falcon)there is Hua Mark Police Station...just after there on the right, is a long, through soi(maybe 26)and this is the other end of Rama IX soi 49. Take a motor cycle taxi from here and ask for Rama IX(pronounced: Param Kow)and just before you get to Rama IX, up a sub soi, you will see signs for Falcon..maybe..before Uomo was well sign-posted.