Orientate the strap so the keepers and buckle face up (same direction as watch)
Insert the strap between the top spring bar and the watch case and pull through
Adjust the watch case close to the 2nd keeper from buckle
Pull the long part of the watch strap back through the floating keeper on the shorter part of the strap.
There you have a working strap!
You can tuck the excess length back into the keepers inwards or outwards