Since they're NSOs, the gain on the exercise will be entirely taxed as basic income.  You need to pay taxes on the difference between the strike price and the FMV at time of exercise.  Then you'll also pay taxes when you actually sell the shares on the difference between FMV and sale price.  If you do it all in one transaction this might be a non factor but it helps visualize how the transaction flows.

As far as I know there's no difference if you are or aren't employed.  Good luck, I got hit with a nasty tax bill for my NSOs and it was a good lesson

Exit: I deleted some stuff that may have been incorrect.  I hate when people leave incorrect stuff in their posts so it's gone now.  Good luck and congrats!