There is lot of way to travel from BKK airport. Limo: located just outside arrival level door 5 > highly priced about 800-1200 Baht depending on which car do you use. Taxi: Easy one: from arrival go to ground level (just downstairs) door 4 or 7 > taxis desk give you a ticket. The price is meter price + 50 Baht surcharge + Toll fee is you use Expressway (not needed at night). Minivan and bus: Cheapest option. first you have to go the transportation center > take the shuttle bus on arrival level door 5 (avoid limo touts) or departure 4 floor door 5. you should see a blue sign Shuttle Bus. Bus runs every 20 minutes 24h.
From the transportation center take minivan #551 to victory monument! (40 Baht) Minivan #551 to victory monument
Note the minivan is NOT convenient if you have large luggage!!! From victory monument you can take bus #12 or 157 (12 Baht) to democracy monument 2 minutes walk to Kao san road.