Bow-making tool: a needle placed into a wooden handle.  The length of the needle determines the loop size of the bow, so you can make these tools in several sizes to make any size bow you wish.
Place a 7mm piece of silk ribbon over the needle; leave a tail, if desired.  
Put a tiny drop of glue on the ribbon next to the needle.
Hold the ribbon down on the tool, and pull the long end of the ribbon up the full length of the needle tool, then push the ribbon down over the needle into the glue, forming a loop.
Turn your tool slightly, and repeat, making another loop.
Continue in this manner till your bow is full and fluffy. 
On the last loop, measure the ribbon against the needle, and cut ribbon just beyond this.  Turn under the end of the ribbon before pushing the needle thru to hide the end of the ribbon inside this last loop.
Add ribbon tails to your bow if desired.  Be sure to cut the ends of the tail at an angle, or fishtail them so that they won't unravel.
If you want to try another method, here's a PDF file with instructions.