1) Get underneath the bumper and take off all the 10 mm bolts and 3 plastic clip things that hold the bumper to the under tray.
2) Pop the hood and remove the plastic clips holding the bumper. Look in the wheel wells and remove 2 more plastic clips.
3) Peel the plastic fender liner back. There should be a bolt holding the bumper behind the fender liner that also needs to be removed.
4) This is the scary part or at least the part that seems scary the first time you do it. Grab the bumper from the edge by the fender and just start pulling it off. You'll hear some loud clicking/snapping noises from the bumper getting unattached, but it's perfectly normal. Just keep pulling and you will figure out where it's attached along the top edge of the bumper and underneath the headlights. Should come off pretty easy.
If you look through the DIY section, there is one there about the bumper removal, which will probably be more helpful since it has pics. Not sure how different the Nismo bumper is though.
DIY: Front Bumper Removal