I cut a square of cardboard in the length I want the fringe. Then, you hold the end of the yarn and wrap it around the card board quite a few times. Then, you cut the yarn along one of the edges and you have a bunch of pieces of yarn.
Then, take a crochet hook and poke it through the bottom of the scarf, just above the cast on or bind off row. Put a few pieces of the yarn around the hook and pull the yarn through - but not all the way. You'll have a loop on one side and then loose yarn on the other. Pull the loose yarn through the loop in the back and then pull it tight. Add it as often as you want along the edges.
Once you've added fringe on each side, fold the scarf in half, match up the knitted edges and then trim up the fringe so it is even.
I hope this makes sense!
This page shows how to add fringe without a crochet hook.