Basic slipknot
Most people tie a slipknot naturally without any thought. As it forms the foundation for many other knots, this tutorial is here just as reference.
Grab anywhere along the length of the rope. This divides the rope into the ‘left part’ and the ‘right part’.
Twist the rope forming a loop. The direction doesn’t exactly matter much.
Pick up one of the ends. whatever side you pick up will be the part that slips. In the picture this will be the ‘right part’.
Make a fold in the part you picked up.
Stick the fold through the loop. Be sure to stick it through in the opposite direction from what the loop went. If the loop had been twisted the other way you would stick the fold through the loop in the other direction.
The finished knot. The right part slides. The left part will not.