if its a standard sight, then you just buy the fiber optic rod of the color you want, use an exacto knife to cut it an 1/8 inch over and use a lighter to melt the ends without burning them. I practiced on a piece of rod before I did my first, the trick is to hold the flame as far away as you can but still get the rod to roll over and "mushroom" enough to stay in the sight.
the rod comes in long lengths and most people who buy some have plenty left over