You need to choose an AC adapter that outputs around 8-9v DC and at least 1.5 amps , lots of adapters say a voltage on the case, but when you measure them, they can be as high as 18v, so beware and use a meter to measure them!!
You can add a fuse as per the link on the first page, but I had a fused adapter plug so didn't feel the need to bother. Also, you can add a power connector as per the link on the first page, but I decided to keep it simple and wire the adapter straight in.
Make sure you test for the correct polarity and voltage before connecting it to your NXT
A bit of hot melt glue keeps the cable in place and some tape works for strain relief. You need to cut out the little cable access hole in the battery cover of the controller (thats what its for)
It only takes seconds to slide the board in and it works perfectly, just remember to turn off the sleep mode on the controller if you want it to work for extended periods.
All in all, cost me nothing to make and took around an hour to complete.
You can come over and visit us at the UK Robot Group website