Hold one strap in each hand.
In order to keep the right strap strained, you can put your foot on it easily.
Twist the left strap from behind around the right one.
Create a loop by pulling the strap across its beginning.
Wrap the strap around the other one once again, this time a little bit further down.
Pull the end of the strap forward through the loop and tighten the knot. Now, you can slide the knot along the right strap up and down!
And the slipknot is done!