Hold one strap in each hand.
Bring the left strap under the right one.
Now, fold the left strap across the right one, creating a simple knot.
The end of the original left strap should be on the right side and in the back now.
Put your left hand slightly further down on the right strap to form a loop.
Then, grab with your right hand from above through the loop to take the left strap.
Pull the left strap through the loop.
Tighten the knot so that it is getting flat.
And the flat reef knot is done!