Start at the very top, french braiding downwards and on your 2nd cross over, add in a ribbon the same way you would yarn to braid, then continue the french braid until there's no more to french braid, then just normally braid and finish it however you would like.
My friends show saddleseat and they use multi colored ribbon. She braids them ahead of time and then will use a rubber band and secure it in with out actually haveing to braid it and ruin the hairs. If your horse's forelock is think this will work.
You hold the forelock (which is usually cut to be very thin) with one of the three ribbons. The goal is to try to fold the ribbon over the piece of hair in order to conceal it. Then you just braid as usual. It helps if someone holds the top of the ribbon for you. You want at least 3 inches up at the top so that once you are done braiding you can tie the excess around the top of the braid to hold it in.