I always start it out with the right side crossing over the left, so that I end up looping it over and down behind with my right hand.
Then I make a loop by folding the right strand under itself (with the excess ribbon hanging down from thumb).
Now I lay the top/left ribbon over the other loop and tuck it up through the right side. *TIP- The key is to make sure the ribbon stays as FLAT as possible the whole time.
Then I pull the loops tight to finish the bow.
Don't forget to fluff the loops of the wider ribbons to make them look full and finished!
You can also form a perfect bow without tying it around something. Just cut a piece of ribbon, longer than you need but not too much to waist.
Being conscience of where the middle of the ribbon is, I make a flat loop (with one half of the ribbon) around the back of my right finger, leaving the long part hanging behind. Then I hold it in place with my thumb and index finger.
With my left hand, I bring the long part up (from behind), up and over my fingers and other loop, to the front.
Then I tuck it up under the right one (just like I mentioned above) and finish it off.
I'm sure to pull nice and tight, and always have to play with it a little bit to make sure the loops are even. Then I trim off excess ribbon at the ends to give it a uniform, finished look.
*TIP- Depending on the size (width) of the ribbon I'm using, it will determine how big or small to make the bow. (They always look best if the sizes coordinate.)