If you are flying out of BKK and return to BKK within 45 days, you can add HND-KMJ-HND to your ticket and pay around 28,000THB for everything. Note that it is a 45-day valid ticket. (No more one-year tickets, which they used to offer.)

You can either go to a travel agent or go to the NH office on Silom Street.

If you ticket this with no stopover in Tokyo, NH will provide you with a coupon worth around 3,000 yen, enough to cover transportation by bus from Narita to Haneda airport. If you are going to stop over in Tokyo, NH will not provide this.

From Narita to Haneda, you can either take a direct bus,

http://www.limousinebus.co.jp/e/index.html

or take Keisei train line. But I think you need to change trains at Aoto station on the way, across the platform. I am not sure on this.

Buses are easier. The ticket counter is just outside after the customs clearance at either Terminal 2 (NH's terminal) or Terminal 1. Subject to traffic, it takes about 2 hours.

HND-KMT is 28,300 yen in full fare one-way. 27,100 yen per one way if you buy a round trip and return (I think) within a week. Tokuwari and Hayawari discounts are both 18,400 yen.

As noted earlier, you can lock in the Tokuwari fare if you book it when it's available and keep it until 20 mintes before the flight. You can just go to the check-in counter and pay by credit card.

If you like, I can book it for you. I need your name, gender and age. That is all. You can also enter your FFP number (other Star Alliance programs included) but this you can also do by calling NH.

Or alternately, I can give you instructions on how to book on the Japanese web site, like which box to check, etc. But this can be a bit risky if you pick a wrong city, for instance.

Also look at NH's Singapore web site... They seem to offer 50% discount on domestic fares if you fly them internationally.

http://www.ana-sin.com.sg/english.htm