natira express

Returning to natira airport from Tokyo - staying near asakusa subway - how do I go about this?
Want to travel by N'ex express.

It's correctly spelled Narita Airport.
If you wish to catch the Narita Express, you need to first get to a station where it stops - such as Shinjuku, Shibuya, Shinagawa, or Tokyo.

Check the schedule on Hyperdia.
http://www.hyperdia.com/en/

When in the day is your flight from NRT scheduled?

All seats of a JR-East "Narita Express" train require reservation. You can try it in Shimbashi Station from 07:00 to 22:00.
No discount is available concerning a NRT-bound ride of a "Narita Express" train.
Your JR station in NRT is either Airport Terminal 2 Station (in Terminal 2) or Narita Airport Station (in Terminal 1).

staying near Asakusa subway

Do you mean Asakusa Station or Asakusa Line?
I recommend you would check in advance which entrance of which subway station is good to access from your hotel.

- Map of subway Asakusa Stations (A-18, G-19)
http://www.tokyometro.jp/station/asakusa/yardmap/images/yardmap.gif
- Map subway Shimbashi Stations (A-10, G-08)
http://www.tokyometro.jp/station/shimbashi/yardmap/images/yardmap.gif
: You see a JR gate of Shimbashi Station in the middle of the map.
: On Toei subway Asakusa Line, Shimbashi (A-10) is the third or the eighth station after Asakusa (A-18), depending on trains.
: A transfer route from Asakusa Line is equipped with escalators.
: On Tokyo Metro subway Ginza Line, Shimbashi (G-08) is the eleventh station after Asakusa (G-09).
: A transfer route from Ginza Line includes a staircase.

Inside gates of Shimbashi Station of JR, you can find a JR ticket office (open 07:00 -22:00), where you can try reserving your seats of a Narita Express train. Then you can go down to Yokosuka Line platform on the basement, and take a Local train (departing at Underground Track 2) to the next Tokyo Station.

NRT-bound Narita Express trains depart at Sobu Underground Tracks 3 and 4 of Tokyo Station.
If the Local train from Shimbashi Station is bound for a further station, you are highly likely to take a NRT-bound Narita Express train at the same platform of Tokyo Station.
If the Local train from Shimbashi is bound for Tokyo, you may need to change platforms to take a NRT-bound Narita Express train in Tokyo Station.
The two Sobu Underground platforms of Tokyo Station are equipped with escalators.

NOT: after Asakusa (G-09)
BUT: after Asakusa (G-19)

From asakusa, the narita express is probably not the best way to go to the airport since it's pretty expensive and it takes time to go to a station where the narita express stops. I think the best is to take the Keisei line fron Ueno since it's much closer from Asakusa (depending where you are in Asakusa you can even go by foot)

Some of the trains running through the Asakusa subway go all the way to Narita airport. Ask the attendant which train to take. It will be faster and less expensive than the Narita Express.