How to tie a Cow Hitch
Double a length of rope to form a bight. Pass this bight up through the ring or around the pole (from back to front). Make sure the bight is nice and wide and sits either side of the long ends that you are not using (standing parts)
Pull the standing parts up through the bight.
How to tie a Pedigree Cow Hitch
If only one end of the rope is going to take some strain you can use a Pedigree Cow Hitch.
Repeat as above. Now take the end of the rope and pass it between the bight and the ring.
Pass it all the way through and then pull on the standing part to secure the knot.