The Carrick mat is an easy knot to tie.
Start with a clockwise bight, to layover with an open pretzel. Run your working end under your standing end. This will bring you to the left side of the flat knot. Run the working end like you are sewing. Over, under, over, under. This will bring you near to your standing end. Run your working end next to the standing end, following it through the knot. You may do this as many times as you see fit.