Are you talking a single 'accent' braid at the end of the bridlepath (beginning of the mane), that we do when showing Saddle Seat?
IMO - do not pre-braid.
We take a very small chunk of hair; just like a pinch of it; divide it into 3 pieces; take your three pieces of ribbon, put it up to the base of the mane (as in base of the hair), with a good 3-4" of it sticking up passed the base. One piece of ribbon to one section of hair.
Have the person holding the horse hold the ribbons really tight and right at the base of the mane. (I'm usually the holder, and I grab the ribbons between the tips my thumb and index finger; and kinda push the tip of my index finger into the base of the mane right where the hair comes out; so the braid stays nice and tight to the base)
Kinda fold the ribbon in half around it's section of hair, shiny side out. and braid. the braid should end up being tight and neat, so think about pulling real tight and close when your braiding (thus why the holder has to hold on tight, otherwise you'll pull the ribbon down the hair!!)
Braid till the ribbon is about the same length as the mane; maybe a little bit longer.
Once done braiding, take one end of the excess ribbon (make sure you cut yourself 3 very long pieces of ribbon, to make sure you have enough; especially for your first few tries); preferably a piece that is kinda 'behind' the other two; wrap it around the braid, then bring it up and then down through the 'wrap' (I hope that made sense LOL)' pull tight; this will tie off the end.
Do the same to tie off the top.
Then, take your scissors, fold each end of the ribbon in half, and cut on a diagonal so you end up with a "V" (an 'open' "V" with TWO tips, not an arrow, with one
) The end should be about an inch or two long. "V" all three ribbon ends at both the top and bottom.
And you're done