Change to a NATO strap
Insert the double-ended part of your spring bar tool into the point where the band is connected to the lugs, and carefully try to get the spring bar free. Mind that the spring bar tool has sharp edges and that it is easy to scratch the metal surfaces of your watch.
Depress the strap to expose the spring bar and then try to slide it out of the indents in the lug. Repeat on both sides of the strap, and be sure to keep all the pieces safe and secure.
Remove the spring bars from the strap you are changing from.
Put the spring bars back into the lugs, without attaching them to the NATO strap.
Prepare the NATO strap by separating the two strap-parts from each other. Pull the longer strap piece up through the metal loop.
Start at the top of the watch case and insert the long part of the strap from the front of the watch to the back through the opening between the watch case and the spring bar. Pull the strap and insert it from back to front through the opening at the bottom of the case. Slide the case to the middle of the strap.
Insert the long end of the strap through the metallic loop at the end of the shorter part of the strap. You're done!