I like two use three semi-coordinating ribbons in different widths.
I take the widest and wrap it around my fingers until I have two complete loops. This ribbon will determine how large you want your bow.
I have no idea what this stuff is called (helpful huh?), but you can find it in the ribbon section of any craft store. It is an inexpensive twine that is some what wiry, but still very flexible.
I wrap it around twice and tie it off. Then spread the ribbon for a desired effect.
I flip it over and tie the whole thing securely in the back before trimming off the excess twine.
Sometimes I top it of with a button or a fabric flower, but when I choose to top it off with ribbon I cut a small-ish length and tie it into a loose knot for a decorative finish.
I put a small dot of hot glue in the front of the bow to hold the knot in place.
Use your favorite kind of clip, (I prefer this kind because they slip easily on top of other bows and hold well in fine toddler hair), and hot glue it to the back of the bow. I try to hold it open so that the clip doesn't glue shut. This can be awkward to hold,...so sometimes I just slip something between the prongs.
Once it is really well secured I wrap the top ribbon the rest of the way around and hot glue it between the prongs. This also secures the clip on better.