Really Reasonable Ribbon has a great tutorial ->here<-
It's for a double loop but you can easily just go around once rather than twice to get a single looped bow. I did get a little confused but how I do it now:
for the single loop bow:
-- Keep about 1.5 inch "tail", you can cut this off later, and hold it between your thumb and the base of your middle finger.
-- Wrap the first loop around your pointer finger as shown then bring it between your two fingers and then go around your middle finger to the center again (like a figure 8 ).
-- Then go around your pointer finger a second time, this will be part of the knot.
-- Bring the end between your fingers below the loops (closest to your palm, this makes the "downward section") then up and over from back to front again, bringing the end through the downward section from the same side as the beginning end to the other side.
-- Pull down, then up, working the last loop from around your finger then keep pulling up and down until the center knot is tight enough to remove the entire thing from your fingers.
If you're using a one-sided patterned ribbon, like a grosgrain, you can flip the bow while you're wrapping to keep the pattern on the outside of your fingers. for smaller bows with smaller ribbon or twine, you can actually use a fork in the same way.
I love how the bows come out. I also have a mini bowdabra that I got from Walmart for $7 that I really love and is pretty easy to use.
Hope this helps!