Start out with two lengths of ribbon. One will wrap behind your mat; the other will be your "bow".
Tie a loose knot in the longer piece.
Slip the short piece under the knot.
Pull the ends of the long piece until it's tied the way you like. You can move the short piece so that the ends are even on each side of the knot.
Tape the ends behind your mat (Robin of My Pink Stamper calls this the "cheater" method).
There it is - a nice flat knot.
Thanks for stopping by today.
Hugs and Happy Crafting,