Ok this one works great.
Take a something you can put two pencils into.
or use other halfs fingers.....
take a length of ribbon and find the centre.
wrap the ribbon round your pencils or fingers.
place one length over the other pass it through the gap of the pencils underneath the ribbon and bring it back over the top of the ribbon to the front, then do half a granny knot and pull tight. now just remove the pencils and you have a flat knot.
http://www.theglittergirls.co.uk/videos.html
look for bowshaper
this way you can make loads the same size.