The constrictor knot starts as a clove hitch then the working end passes over and under again. A clove hitch has two pieces parallel around your post(or whatever) with a diagonal line across the top of them both, take the working end(probably shortest) and pass it over and under the opposite piece then pull tight!
You are effectively tying a half hitch( the knot you start a shoelace with) under the diagonal piece.
A lot easier to show than explain
There are a couple of ways of making the knot so someone else may have a different technique