First, install both spring bars onto the watch lugs. You can do this either with your finger or using the spring bar. Then, pull the long part of the strap out of the floating keeper, so the two layers of the strap are separated.
Now, orient the strap so the keepers and the buckle face up (same direction as the watch face). Begin on the top of the watch (the end of the watch with 12 o'clock). Insert the strap between the spring bar and the watch case, and pull through.
Next, you will do the same thing on the bottom of the watch (the end with 6 o'clock) and pull the long part of the strap back through, this time from the bottom. Again, you will pull the strap in between the watch case and the spring bar.
Last, pull the long part of the watch strap back through the floating keeper. Pull it all the way through, and snug the floating section of the strap up to the watch case. Adjust the watch head on the strap so it is centered between the keepers, and you are all finished.